[anta40]

Perhaps build simplicity is also an attractive point?

Basically a rather simple Makefile, gcc, qemu, xorriso...

I see lots of open source hobbyist OS projects require you to build your own GCC, which is not fun. Xv6 works with the standard gcc (provided by distro package manager). On macOS: just install homebrew-i386-elf-toolchain, and tweak some lines on the Makefile. Done. You are ready to build Xv6, which should take about a minute or less.