> I do not know what the next-best single-user, single-process, non-bloated OS would be to run on modern hardware that still has some reasonably modern software and can be used for distraction-free (hobby) development the way FreeDOS could.

Not sure why would you want a single-process OS on modern hardware, but there are some alternatives that run much less things on the background than regular Linux: Haiku, FreeBSD, NetBSD, OpenBSD, or some lightweight non-glibc, non-systemd Linux-based like Adelie or Alpine.