Re. some of the highpitched digital noise. The STM32 chip is actually a little too good at regulating its own power consumption. So there is a transient in current draw when the doing periodic things (like processing audio in the callback). This causes a slight lift in the GND signal that can create this digital noise (most audible at the callback frequency).
A bandaid that works when doing fairly trivial DSP is to increase the callback frequency to something outside of, or at the upper limits of human hearing. Obviously this is not always possible.
What we did on the Petal, and Field hardware (similar to on the Patch, etc.) is to use a DC-DC Isolator to create an isolated 5V supply that powers the Audio parts of the circuit (Buffers in this case). The daisy is then powered from 9V directly, keeping any noise on the 9V power brick instead of the audio path. (On the Daisy Patch we did the opposite – powering the Daisy Seed from an isolated supply, while the rest of the circuit and eurorack power stays clean).
The PDS1-S5-S5-M has worked well for this (5V-5V isolator), as has the PDS1-S24-S5-M (24V-5V isolator – used on the Daisy Patch)
The circuits for both the Patch and Petal are up on the Hardware Github Repo