Here's a tool for different keyboard layouts that you can do analytical tests with to see which one you like.

When I get around to making a custom keyboard, I want to try MTGAP.

Here's an awesome split keyboard GitHub repository with a lot of information that may help.

I'm not sure how ergonomic this is, but it is a very unique design that enables insanely fast typing speeds: CharaChorder One.

On second thought, I would not recommend the CharaChorder, especially due to poor customer support, the double key press bug, the closed source firmware, and the fact it is still $299. I would be interested in an open source alternative, even if I had to build it myself.

Charachorder did respond to the video saying they fixed most of the issues, but if the device still has closed source firmware, that would be a dealbreaker for me, especially since you cannot remap keys at the firmware level.

I'd go for something slimline. I don't know how people use chunky mechanical keyboards.. That said, there needs to be some clearance. I used a macbook with the butterfly keys and that was attricious.

I used a Cherry ultra thin usb keyboard and it wasn't too bad. It's not ergonomic, it's just thin like a laptop keyboard so there's not much distance to press down.

Have you considered that it might not just be your keyboard? Four things that made a real difference for me:

1. An upright mouse.
2. Dictation software. It takes a while to get used to and putting on a rhotic accent can help if you don't already speak with one. The one built in to OSX is better than the one built into Word, but it may depend on what you're writing about. Expect copious editing – which is where the upright mouse comes in handy.
3. A lumbar support cushion. This will help your posture so there's less strain on your neck, shoulders, elbows, and wrists. Mine is cylindrical. It attaches to my office chair. You don't have to get a cylindrical one but don't get one of those cheap mesh things as they don't provide any support. You could try wedging a rolled up towel between your chair and lumbar spine to see if it helps before forking out.
4. Set up your office chair so your feet are flat in the floor with your knees bent at roughly 90°. Sat upright, your shoulders should be relaxed and your elbows should be open at roughly 90–100° and your wrists resting on either a dedicated support sponge thing or a rolled up towel. I had to cut an inch from the desk legs to achieve this; otherwise either my knees were too low below my hips, my wrists too high above my elbows, or my traps were tensed up. When you're sat upright, put an arm straight out; you should just be able to touch your screen. Make sure the screen is titled upwards and the top of the screen is below your eye level.

Hope this helps. It makes typing much more comfortable.