There is no need to use a compute module, if what you want is a smaller footprint this is a good starting point https://github.com/okyeron/fates
It uses a normal Rpi 4 and the majority of it’s connectors but breaks out some of them to the back and adds the screen, encoders and DAC/ADC
Maybe there could be a slimmed down version of the dac/adc + zynscreen + midi circuit to get a slim build in one pcb + rpi or maybe 2 pcbs + rpi.
I myself tried this slimming down concept on mine, getting somewhat of a slim solution (the cabling an the midi+extension pcb were the main things i couldn’t get any thinner), and i definitely see a point in doing something like this but still supporting the DIY way.
I was thinking that maybe the midi circuitry could be integrated within zynscreen and maybe make a breakout for the extra boards if you needed them (As to avoid the 40 pin cable as much as possible).