I have read the reason they are available is because the die is very small, thus they use relatively little silicon an are easy to make. Not because they are crummy and they made too many and no one wants them…
The small die size may be related to the no internal memory, so you have to pick one - available or internal memory. Using an actual pico board seems like it mostly solves that problem, because they are only $5.5 usd each and they have some other good stuff on them which you’d have to pay for if you made your own board anyway. In other words, a HAT may not be a bad idea after all. Suppose it’s $17 for both after everything.
However I did not have too bad an experience with the STM32s and I am generally agreeing and thinking that a G431 based board is still the most advisable path, as a single board for the community to adopt as the flagship.
You obviously know way more about the details of microcontrollers, but I guess where I’m coming from is that features that are supposedly wonderful but somehow have never been harnessed are not that interesting to me. For example, Arduino has been around for a long time, and I did see some stuff about people throwing around the idea of making non-blocking I2C code, but no one has ever done it. So here we are.
One valid approach may be to make a list of libraries for arduino that need to be written at a low level to make simpleFOC more practical. However this would I assume end up being specific to hardware. If we flocked around say the lepton 3.0 this may be worth doing, as what gets done stays done and is multiplied by the quality of the board etc.
As often happens, we look for a way to squeak by and make do, and two cores or even two MCUs does appear to be helpful in making that happen. I do think it is a sound concept on this subject, as it eases complexity and debugging and timing issues.