I highly recommend Axoloti. It has all the connectors ready to go, is easily expandable to write custom code but has all the boilerplate stuff and basic object

I highly recommend Axoloti. It has all the connectors ready to go, is easily expandable to write custom code but has all the boilerplate stuff and basic objects, so you can get creativity quickly without having to write and debug an envelope generator and filter, etc. Or if you want to start from a lower-level: Teensy 4.0 is amazingly fast. Teensy 3.6 will suffice. Nucleo STMH734ZI2 is great, too. Nucleo STM32L476RG is not as fast, but good enough for single voice synthesis or basic guitar effects. STM32 Discovery still holds up well.