Things go on in life, but it's been quite smooth sailing regarding this little one.

Firmware development was a breeze — Puya provides a library package that is largely compatible with STM32's HAL in terms of concepts and conventions. I2S audio was emulated with DMA-driven SPI plus a timer outputting PWM. The entire instrument works as intended. I'll record a video soon!

One major setback is that the 20-mm 8Ω 1W speaker is proving inadequate, causing noticeable distortion in the sound — the issue disappeared when I replaced the speaker with a larger one rated 4Ω 3W. Seems like something to be thought over.