@all: Thank you very much for all the comments!
To give something away, the most promising way for me would be to sell ready built and tested gadgets because then I'm sure you get a working item. The risk of self-sourcing and soldering SMT would be that troubleshooting is hard since you can't plug out the IC off a socket, changing components isn't easy, populated components are hard/impossilble to identify... and so on, you know this. Selling PCBs would of cause be easy to I-know-what-I-do-guys.
Please keep in mind if you self-source you will need a programming/debugging dongle from FTDI ("VNC2 DEBUG MODULE", approx. 25 EUR) and the windows-only toolchain from FTDI (free).
Eurorack: you can simply put it behind a 6 or 8 HP panel, skip the DCin connector, and plug it into the 5V rail. I consider to arrange a 6x2 eurorack pinhead.
Actually I wonder to change the 9V DC barrel plug to a USB micro connector to use a simple/cheapo USB power wallwart. Then the 5V regulator can be skipped. What do you think?
MIDI IN: Software wise this would be easy since the UART Rx is not used yet. But we would need the circuitry with optocoupler and a muuuch larger front panel... A solder point is easy but it's neither fish nor fowl... More than 1 MIDI DIN I/O I don't like because I only have 1 UART.