Interesting, thank you! I'll take a look at Xemomai, but it seems like the options are very limited. I'm building a TVC rocket, so hard real time is unfortunately a strict requirement.
Xenomai might be fast enough with some coprocessing added if necessary. It makes your RT threads have essentially "normal" POSIX RT api, but runs them on separate kernel that can respond to things like interrupts much, much faster (including stable timeslicing)
For something like a thrust vectoring rocket, I'd stick to a sub processor running an actual RTOS or bare metal code.