Can you self-host, or are you looking for another online service? Facebook Groups is basically a forum when it comes down to it, and any forum software can do what you're asking. I really like Discourse. You can self-host it for free (well, whatever your server costs), they'll host it for free if you're an open-source project, or if you're a legal non-profit you can get 50% off their hosting for $25-50/month.

Comments are pushed out by the community's server, they're not pulled in by yours. So if you're missing comments from communities hosted on lemmy.ml for example, it may be that lemmy.ml is overloaded and not sending out comments to the fediverse properly.

The other common issue with missing comments and posts is misconfigured language settings in your profile. You need to make sure at least Undefined and English are both selected, lots of people only have English selected which will make a lot of posts hidden.

I would describe Apollo as an accessibility app in the sense that the regular Reddit app is unusable.

"The content" on all Lemmy instances is the same. There is no account migration, but you can just sign up on lemmy.ml. If you already had an account there and you want it back... I don't know if it's possible for an admin there to restore it, you might have to get in touch with them.