Littlekeyboards, and boardsource.xyz both have the parts (and assembly services if you don't want to bother).

Keycaps can be hard, chocs in particular don't have a lot of good caps. MBKs are the best feeling ones (in my opinion). They just announced ones with legends and they won't be available for months. There are other options with legends available as well, that are closer to traditional laptop keys.

https://github.com/diimdeep/awesome-split-keyboards - A list of splits, not all support lower profile keycaps.

Reddit has the https://www.reddit.com/r/ErgoMechKeyboards/ community, who might be able to point you to more information, including a layout comparer.

Board: You might try https://flashquark.com/product/gb-mitosis-wireless-split-erg... and seeing if you can get custom switches installed. I believe the pin outs are the same, so it would just be a matter of sending them switches they don't currently sell, if they agree.

For keycaps, Signature Plastics is where I buy blanks. DSA profile will be your best bet, given how strange split board layouts can be.