I'm looking to William Lamie's next project after ThreadX, the PX5 RTOS. The POSIX API won't be an afterthought.

https://px5rtos.com/

Yes, I have seen this. Looks interesting.

Its certainly targeting a more enterprise audience (much like ThreadX was originally)

You have to pay to use it, price unknown other than maybe >= 5k$ per year?, and it looks like it will be prohibitively expensive (for my usecases at least) to have source access.

To me, those make it nonstarters.