I was thinking of posting something along these lines, nice to see someone else also aware of this niche!
Software point of view, you'd like to use QMK or ZMK, and KiCad to design the hardware (PCBs). Reddit's r/olkb and r/ergomechkeyboards are great places to start!
Bonus: QMK supports trackpads and nipple-pointers too! (Not sure about mice though)
This is something I've been interested in lately as well & came across this video: https://www.youtube.com/watch?v=M_VuXVErD6E
It's targeting a split keyboard, but I think it details the general steps involved pretty well.