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Have him stab the mayor who's evil because he's greedy and selfish and borderline abusive in trade-deals with neighboring regions but is otherwise beloved (and has rewards heaped on him) because he's so good at actually keeping order in the town and keeping their goodwill (although probably at least a little bit through some passive-aggressive blackmail). That's always fun.

Yes and no.

If he'd gotten powers from the divine oath-giver he'd be a Warlock or Cleric, dependent upon the nature of their relationship and the being's powers.

If he got the powers himself from his absolute rigid dedication to his oath, then he'd be a 5e Paladin (I prefer "Dedicant" or "Crusader" for which Paladin should be a specific Oath but that's a different conversation).

Otherwise in older editions he'd probably just be a devout warrior.

For those older editions he'd only be a Paladin if the oath he held to was far more specific and arguably he and several of the other hobbits were a bit too quick and dirty for. Particularly during the era of Racial restrictions to classes which didn't allow halfling Paladins. (Assuming halflings to hobbits is 1:1 in all settings, which is far less consistent over time.)

For how a generous DM might work around that in older editions sometimes, I'd look to BG2's Mazzy Fentan: https://baldursgate.fandom.com/wiki/Mazzy_Fentan

...I miss when Time Stop let you cast attack spells...

Have you considered an anti-magic field area of considerable size? That's hardly a good continual response but it may allow a more challenging encounter, especially if the field can be briefly turned off by characters expending spell slots into a single dangerous spot while being attacked. Since it prevents Divine smites and the bonuses. (Someone could 5e-argue that probably, since its pretty nonsensical with class abilities at times but I'd just overrule it.) You can even use it with Mindflayers or other psionic aberrations, since arguably their abilities can still work if you rule they can. (Psionics don't necessarily equal magic unless you set some other precedent.)

Other concepts are to add hazards and chaos causers to battlefields. Floating clouds of glittering fog that reflect spells to random other targets, negative-energy/undead quicksand bogs that leech endurance instead of doing damage (and whose saves are to avoid getting stuck not to avoid the drain), unsteady floors to drop out and separate parts of the party, or lair effects that randomly teleport characters back to earlier areas of a dungeon, forcing them to run back through to get back into the fight (don't overuse that one but if there are traps earlier its a great way to force them to ignore traps in their rush).

Also just tasks to do while being attacked. The paladins + cleric can defend well together but force them to be separated into different regions doing a task and you up the tension (even if you don't up the danger).

You can also try bringing forward older monsters that undercut benefits/items. Black Puddings, Rust Monsters (Or their papa Annihilators), or port over other monsters like Magerippers or Spellweavers from 3.5.

Traps can also be good, since they may be taking 1/4 damage on a save, but if the traps also cause inconveniences or force them through alternate, and slowly more damaging paths (like crawling through a stone brambled tunnel instead of taking the other tunnel because it caved in and almost crushed them), they can still build up and be valuable.

Also if they're incredibly proficient in combat, the enemies probably know that by this level. So you might have to start attacking them legally or socially, depending on the situation. Or just start having foes avoid them. Make them burn resources to set up engagements where foes can't run or attack them when they're on their back foot relaxing with assassins in the bathrooms, poisons in the bar bread, false accusers of horrible crimes on the streets and in court. (Depending on who their foes are of course.) You do that so that their character get paranoid. Start trading things like spell slots, the benefits of sleep, or close allies to try and defend themselves, so that they're weakened before they even touch something like a dungeon. If they party is just too invincible in combat as it is, don't just attack them there, let them know that existence is sortof a threat. (And as before, how much you use it is important an should be informed by session 0. You want them tense and excited, not miserable, and "the world as the DM's weapon" isn't necessarily the right way to approach it but its a nice tool to have in the chest. Hammer finds nail and all that.)

Simple rules that can describe almost every situation are also rules that over-generalize characters to the detriment of options (everyone's noticing the same things, instead of perception allowing more observant characters to do what they could do), over-include the player's capabilities in place of the character's. (Players conversational skills failing to match with those of the character they intend to play), overly abstract what they describe (a monster's "power" or a character's actual abilities meaning something in adjudication but nothing consistent/concrete enough in-world), or demand a DM adjudicate without reinforcement or restriction (In the absence of rules every corner case ruling risks the danger of turning the table into a debate between PCs and the DM, inviting rapid ends and either producing embittered DMs or embittered players* - especially under the "pack it up" approach the video suggests - and helping to increase combative tables in the future.)

The games that OSR takes inspiration from did a lot right in their mortal power-level, reasonable growth, real risk of danger, and humanistic tones but if you're trying to sell me that the growth of rules that followed aren't a direct result of weaknesses in those games? I don't think we'll agree.

*The "Dorkness Rising" problem, for a slightly more light-hearted allusion.

Ah, see, I was preaching to the choir. You're on top of it already. =P

That sounds insanely dangerous for like a bunch of reasons. Like "that's a Planescape campaign by itself" dangerous. So, yeah, let us know what happens when you get there. Sounds like fun. =P

Killing him altogether seems pretty epic level, like level 25+, given that he's a deity. (But your DM could be ballsier than me, lol. Killing an aspect of him to weaken him for a bit seems more my speed.)

Alternatively you could try shifting goblin worship in localized communities to another deity. Maybe someone like Kikanuti (since I imagine getting them to worship someone like Tymora immediately might be too much of a jump?) or some other goblin. (Were Konsi to be more arrogant, I'd suggest her. =P) Kill him slowly, death by 1000 cuts of lost faith style.

For D&D I can also think of Lolth, Tasha//Iggwilv (depending on your interpretation of that mixed bag of lore threads), Ravel Puzzlewell (a hag so powerful or madly knowledgeable she has aspects across the planes from the Planescape setting), and Loviatar the Mistress/Goddess of Pain.

There's also existent queens like Queen Mary("Blood Mary") that oversaw religious purges that you could take inspiration from. (Zealots are rarely reasonable enough to be bargained with.)

You could also just take a historically male example and invert the sex and not make a deal about it. See if your players even notice.

It is worth noting that the -Con score was a 3.X house rule but Pathfinder 1e raw. It was just -10 otherwise, which could get pretty punishing if you were dropped by bad luck.

5e's up-and-down approach to unconsciousness isn't really an ideal resolution, although making them gain levels of fatigue almost makes it functional.

Remember everyone, doppelgangers can read minds, including the minds of someone they capture before killing. So learning to act like you convincingly is more a function of time, and the fine details of manipulating your friends is then reinforced by reading their minds. Their biggest hangup is probably how lazy and selfish they are. They know what you'd do, they know how convincing their acting job is on your friends, and they may have even watched you, but they still aren't all that motivated to do it all the time. And I mean, hey, if they get caught then they just kill the person who confronts them if they can and run if they can't.

But yeah, a doppelganger would never pick Konsi to imitate for very long. She works too hard.

Okay, chad faced joke aside (which is what I was going for there and couldn't undercut it with =P) I do prefer more simulation based ruleset which sortof demands that at least some basic rules of physics are able to be mapped between reality and the gamestate. Abstractions can exist, and explanations can be provided (like the idea of a potion only being a mouthful/shot in quantity and/or size), but strain of an increasingly divorced rulest from the actual narrative is always a problem and should be avoided, imo.

No

Looking at some Pathfinder 2e stuff, you could maybe use a dimension. The Sphere of Annihilation doesn't really annihilate you, it forces you to a different contiguous dimension with strange physics and alien creatures/growths. Off the top of my head my idea is something like:

1 - A world of boney growths that work like a cross between coral and fungus, with microorganisms spread as sporelike beings (that can grow in the players). The other dangers are lack of normal weather, clean water resources, and giant insecticide creatures (predators and prey) living on, what's revealed to be a ball of rotting flesh and impacted bone. A literal world made of the the banished and slain by the Orb.

2 - The world is littered with the discarded possession of the dead, lost scrolls rotten by strange conditions, weapons rusted or eaten by the inhabitants, and fragmented journals and maps carved into chitin and bone that direct newcomers towards brutal settlements of inbred survivor clans. (Your choice if they're hostile, creepy, friendly, or any insane mix of the 3, but they should probably be powerful to survive)

3 - Magic is wild here, gravity strange and inconsistent (and tiring to continuously fly in for non-native creatures)

4 - They can catch glimpses through to the real world by painting the inside of skulls from their world with the blood of insect monsters and staring through its eyes (forehead to forehead). (They can see their friends journeys if you want, or maybe just random glimpses of whatever plane/setting is closest)

Then have one group be on a survival journey in an alien world and the other a "how do we get them out" journey. You could maybe even imply the lich could know, complicate the issue by requiring them to confront him once without the phylactery (or with it as a bargaining chip) for information.

As for the Grim Reaper? You could use him as an example of the dangers of the place. Have him do something (cut through some of the dangerous coral-fungus to get to a player or something) and merc him on the spot as a warning. You don't need to necesarily take it easy on the party from there, but really put the fear into them that this place should not be messed with (and maybe make them afraid that their friends are trying to save them).

And for the character who cast Planar Anchor? Maybe kill him? Send his body, dead, to the alternate dimension but his ghostly soul stays with the other group. Until he's resurrected have him be able to occasionally hear the surviving players in the alternate dimension.

Not sure how much customization on the alternate world monsters you'd want to do, maybe just use regular giant bugs with far realms templates or something (not sure all what Pathfinder 2e has) but you could give the remains of them extremely weird powers that might later come in handy. (Like strange multi-planar scent based stuff which could be useful in tracking down the phylactery later, just in case they do trade it). Although you could also imply that they could be importing an infection coral-fungus thing into their real dimension.