I use my zynthian in a very similar way to you, as an expander for my digital piano. Just curious to know what screen you have gone for and what input methods you use?
I have a 4.3" DSI (ribbon cable directly into the RPi4) touchscreen and use the screen’s touch facility and a mouse for input. I’ve often thought about adding 4x rotary encoder knobs but never properly gotten round to this. I’ve also tried using a Korg Nanokontrol for input. The nanokontrol works fine and has almost too many ways it can be configured but it’s probably overkill for what I need.