I wrote a software synth myself with the intention of running it on Raspbery Pi 3 / Zero 2. Those are actually quite capable processers; sound synthesis requires very little RAM, both code and for maintaining state, so everything fits in the rather tiny cache. But at the same time, while these Pis use "little" cores, the maximum throughput of NEON instructions is actually the same as for the corresponding "big" cores like the Cortex-A72. With four cores, you can do in the order of ~10 GFLOPS 32-bit FMA instructions. With a sample rate of 96 kHz and 32-note polyphony, you theoretically have a few thousand FMA instructions per note to spend.

I discovered by pure chance the vast ecosystem of iOS synths apps. And I was absolutely blown away. 90+% of the synths of the past century are available for ~5-20€ each. Connect that to a basic KORG MicroKeys Air [extra cheap, but includes Bluetooth]. And you are the next Jean-Michel Jarre.

[Note: and the amount of tutorial videos on YouTube is huge.]

theres also things like the M8 tracker from Dirtywave which uses a teensy board and sounds really good for a chip of that size

Typical how they first let Korg do the talking, when actually, Korg is much slower to innovate. With one line, the article mentions Tasty Chips synthesizers (who are friends of mine), but as far as I know, they were actually the first, just check their Kickstarter campaigns which go back ten years.

Note that using RPi is not all sunshine and roses. There were times that compute modules were extremely hard to get.

There’s quite a few Eurorack modules that use the RP2040 too

What if instead it ran on a macbook which you plugged the keyboard into. You could still make the software open source if you want.