I'm using a Teensy 3.2 for prototyping because,
- It does fast signal processing because its processor implements Cortex m4,
- It's a decent Arduino-compatible experimenting board,
- It seems to be power-consumption efficient, using much less power than Teensy-4, Raspi, etc.
This project is 50/50 hardware and software, but the Hackaday.io classification makes me choose just one. I'm thinking that the software (programming) will take more work than the hardware. I'm just going on my knowledge of commerical musical instrument tuners --- it's taken a number of years for them to get "good". And highly rated ones cost over $100 US. So I'm assuming that the programming may be tricky. Especially "locking on" to the pitch being played, and being useful as a tuner, not simply a frequency readout.
I'm looking into trying ML with TensorFlow light to determine whether the incoming sound is an in-tune note.