Things were getting really messy:

So I invested some time in a much needed cleanup:

… and made this sketch controller:

It still feels limited – I only have 12 analog inputs (6 per Arduino), and I’d like twice that. I’ve ruled out Multiplexers, so for now it’s either more Arduinos or this. I’ve ordered one but won’t have it for a couple of weeks. With it, I can read sensor values straight as midi.