Hi Matt,
I still haven't gotten around to this one yet (its rather down on my rather long procrastination/project list... and I keep adding to it...) but in my head my MCU+USB was always on a separate board from the main PCB.
The reason is that I was probably going to use a standard 'black pill' board rather than designing the main PCB to cater for the MCU and all its peripherals etc - I would hate soldering some of that stuff including the USB-C connector...
It should add minimal 'thickness' to my design and I'm not aiming to make mine super small or tiny so I could always place it away from the main key matrix if needed (if I wanted to avoid 'thickening).
I don't think I've shared what I had in mind for myself...
My plan is for a condensed 'keyboard' + trackball + numpad as indicated by the rough arrangement of keycaps shown in the photos below. They keycaps are DSA and XDA profile and I plan for them to be paired with the Kailh choc v2 key-switches also shown in the photos below - ending up with a 'low-medium/flat'ish' keyboard that's not quite standard, not quite low profile...
I've grown semi-accustomed to my Keychron V3 other than running out of batteries every week or two confusing me why the keyboard isn't working hence the condensed layout that follows it. The trackball may be detachable from the main keyboard but will sit directly on the right of the main keyboard. I really miss having the numpad to type in IP addresses, lots of numbers, and various other things, so the numpad will make a come back and sit on the right of the keyboard and may also be detachable.
I was thinking the main controller will be on the main keyboard, with only several connections/pins required to talk to the trackball part (to the ADNS9800 and a couple of buttons), as well as the numpad part (small matrix there).
The lucky last photo is for Lego moulding any new keycaps I may need ;)
So I have almost all the bits, the controller, the key caps, the key switches... just missing the PCBs and 'case' design that will go with it (thinking of thin aluminium panel... was planning to try
www.quickpanel.eu )
Someone has already merged in ADNS-9500/9800 code into QMK so I don't even need to worry about porting my own old crappy code ;)