I have little experience in that sort of thing, although my own PCBs came out well soldered by JLCPCB.
Now I don’t think JLCPCB is suitable for a proper public GB as they have a lot of limitations.
But for a private projet they could be used if some hand assembly of a few components (like the USB C connector) is acceptable.
Also having a few PCB prototypes in hands before launching something more official is a good idea IMO. It allows to find potential problems (like fitting in your case design) and ways of improvements.
I can try to find a little bit of my time for that if you need.
For medium to high volumes, probably other manufacturers would be better like Elecrow(used by @jmdaly in several projects), PCBWay, etc…
This part is unknown to me, and raises many questions other than strict PCB electronics design.
One for example is having official QMK and VIA support (@jmdaly looks to have a working firmware in QMK repo, but in the handwired directory and no VIA support as it looks).