I started a new thread to keep the topic clean.
Just a fixed idea…
I’m not sure, but perhaps you can use I2S to connect several raspis in series and add up the audio data until a sound card with a DA converter sits at the last raspi and outputs the data? If you also have a parallel data connection (serial or better SPI) between the raspi’s, you could have a master that receives the MIDI data, distributes it to the raspi’s and outputs the added audio data via a sound card.
My ooooooold Roland-D110 had the feature, as soon as it ran out of voices, it forwarded the data via MIDI to a second D110… you could do the same in a ZynCluster.