The biggest issue is power supply…
I’ve tried keeping pi’s alive on USB battery packs and they can be at best marginal once you add a screen & an audio card and a USB connected and powered keyboard.
If anyone has a better experience of this then please relate as it as point out a tremendously laudable idea in many ways.
A Pi Zero would probably be a better target point. I did try and zynth one a fair while ago but dont’ remeber too much success, but things change.
As far as an external zynth goes I would skip the audio out level completely and put a hifi berry amp on the back of it and run the whole rig off twelve or more volts (small car battery? or similar … PV powered current store…? The last would seem to make sense for public use. (Charge all day, play all night . . . ).
Where this does present a problem is audio in ( mics,guitars,violins,church bells,strange gloopy noises in tunnels…) both for your self and others who turn up bearing a 1/4" jack of some sort. But having seen the Pi4 cope with a Behringer 1820 I can’t see why we can’t think about multiple low latency (audio cards) ins and outs in a zynth…
I can squeeze my audio rig ( , powered by a 12 Volt telescope power pack) with JBL control Ones into a rucksack and have cycled it ten miles without discomfort so the zynth certainly does this job very well…
The work of @dhrupadiya is also to be commened in this arena…
But basically, Provide a solid power supply !! 