Been ages since I had to recompile a Linux kernel to deal with hard real time (via RTAI) but I recall emc2 being a great alternative to all the fussing around recompiling as some one did all the work for that.
I also recall using this resource . Eventually I just made a class for the threads I was using to wrap POSIX and RTAI calls for periodic tasks and chose which was the underlying method on a compiler flag. If I was on my desktop I could proof of concept most things in POSIX and then test on the RTAI machine. If I need to revisit this again I may dust off my old class and add freeRTOS stuff to it so I can prototype on Linux then try to squeeze it on to an esp32.