Tainted in that the kernel and ZFS have different licenses. Not a functional impairment. I have no way to check to check a system not using ZFS. For my use case, Debian plus ZFS are PVE’s principal features.

I have synapse server running in docker on a VPS and it’s been pretty reliable. At my office I use it as sort of a self-hosted Slack replacement. For our use case, I don’t have federation enabled, so no experience on that front. It’s a small office and everyone here uses either Element or FuzzyChat on desktop and mobile. It runs behind an nginx reverse proxy and I’ve got SSO set up with Authentik and that’s worked very well. Happy to share some configs if that would be useful.

Have you by any chance documented your PMG set up? I’m also a very happy Mailcow user and spinning up PMG is something I’ve been meaning to tackle for years so I can implement archiving with mailpiler, but I’ve never really wrapped my head around how everything fits together.

Ceph isn’t installed by default (at least it hasn’t been any time I’ve set up PVE) and there’s no need to use ZFS if you don’t want to. It’s available, but you can go right ahead and install the system on LVM instead.