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.