I hope it is going to be ok. Automattic has been involved with matrix before. Time will tell, but I’m more hopeful than afraid.
I tried to compare some backup solutions a while back: https://hedgedoc.ptman.name/kket4uo9RLiJRnOhkCzvWw#
Forget about docker. Run caddy or some similar webserver that is a single file next to the assets to serve.
How full is your ZFS? ZFS doesn’t handle disk filling and fragmentation well.
Linux is quite lightweight. Pick a distro that doesn’t run a lot of stuff by default. OpenBSD only runs sshd exposed to the network, AFAIR. Debian probably does the same. But really, the lightness comes from what isn’t running. NixOS, fedora, rocky, alpine are all decent alternatives.
Quite far: