You can turn an old Guitar Hero accessory into a USB MIDI controller for your synthesizer! Wii accessories use I2C to send all of their data, so we'll couple the Guitar Hero controller with a STEMMA QT Nunchuck Breakout and an Adafruit QT Py to read all the fret buttons, whammy bar, strum bar, and joystick data.
You'll be able to send MIDI notes, chords, octave changes, whammy bar pitch bends, and joystick CC messages to any software synth or a USB MIDI host-capable synthesizer.
Wii Guitar Hero Controller
You have one in your closet. If not, ~$30 on eBay, less at a thrift shop.