My work on splitting out individual prototypes of the USB hub, serial adapter and sound card has paid off! I have working prototypes of all three, and a lot more experience with what support components are necessary and which are just “nice to have.”

Separated sound card and serial prototype boards

Sound card and serial prototypes in use

Based on this new experience, I’ve started reworking my original design. I also tried out JLCPCB’s assembly service, and while they probably can’t build the whole thing (don’t have all the parts I need in stock), they can at least get all the teeny tiny 0603 resistors and capacitors.

JLCPCB design and physical board