> there is even a debian variant using a BSD kernel.

it's dead as of july 2023: https://www.debian.org/ports/kfreebsd-gnu/ :

> The development of Debian GNU/kFreeBSD has officially terminated as of July 2023 due to the lack of interest and volunteers. You may find the official announcement here[1]

here[1]: https://lists.debian.org/debian-devel/2023/07/msg00176.html