[panic]

FreeBSD satisfies all your criteria (all the BSDs do, really). It's pretty similar to Linux, though.

[prima-facie]

I second FreeBSD, it's what I'd use if Linux were not available. Maybe look into other BSDs like NetBSD or OpenBSD.

NetBSD even runs on a toaster if you have any lying around :-) https://www.embeddedarm.com/blog/netbsd-toaster-powered-by-t...

[samoright]

I am inclined to try either FreeBSD or OpenBSD.

FreeBSD comes with the bonus that it is supported on some cloud and VPS offerings, so whatever I learn can come useful for practical purpose.

But OpenBSD also seems like a great idea because I have read great things about its security and documentation.

I am not inclined to try NetBSD because it seems to offer neither of these advantages.

Is my assessment right so far or very misguided?

Is there anyone who has used at least two of these three BSDs? What do they recommend?

[zzzcpan]

I have experience with all three, but I disagree that you can take away a lot from them after Linux. An OS from a user perspective is essentially a package, configuration and service manager. And to learn something new and valuable in either of those categories you'd need to go into very different direction. Like docker, nix, guix for package and configuration manager, erlang supervision trees for services.