The best build log I've ever seen for a split keyboard featuring a trackball is the Oddball 2. I discovered it yesterday. It has a parametric design that's editable in OpenSCAD, so if it's too small for you, adding more keys should be easy.

https://www.reddit.com/r/ErgoMechKeyboards/comments/kxsniq/o...

https://atulloh.github.io/oddball/