Look into rump kernels; NetBSD is an interesting vehicle for research into virtualisation, micro VMs, also device drivers (the framework is said to be very eleg

Look into rump kernels; NetBSD is an interesting vehicle for research into virtualisation, micro VMs, also device drivers (the framework is said to be very elegant). I was also surprised to find out how simple it is to cross-compile the kernel from a different OS. You can also look into pkgsrc; NetBSD is its parent project and pkgsrc remains very portable across many "alien" operating systems, from QNX to macOS. NetBSD was also forked into, and retains many similarities with OpenBSD (the latter is pretty decent for both server & workstation; it is also my favourite OS of all time, even if I don't get to daily drive it). I'd like to give NetBSD a proper try in this area to get my own opinion.