I built an x86 kernel 25 years ago at this point, this time around I chose RISC-V because it's much simpler to get it running, and speed of iteration is paramount on a prototype. There's less documentation but much less cruft. I eventually would like to try porting it to x86, and also to run the same programs — unmodified — on Linux. My first prototype was on Linux actually, but trying to figure out how to fit userspace preemptive scheduling with async IO/io_uring and the complexities of signal handling was literally more complicated than just writing a kernel.

Thank you for the links! I'll have a look when I have a moment in the next weeks and I might send you an email to discuss this further