I wrote a toy synthesizer for the ESP32 where I used STK for the bulk of the synthesis. That was pleasant endeavor but the real pleasure started when I wrote my delay effect. I had so much fun adding features like multiple playback heads with separate feedback and volume.

Another fun thing to implement was the apregiator and something that I call a scaler¹, I learned so much about music theory while doing this.

1- There is probably a name for that effect where you choose a scale and if the effect receives a note outside, it outputs the closest note in the selected scale.

edit: I forgot to add that your plug-in is awesome