"It is also worth mentioning that Proxmox uses ZFS"

No it does not enforce ZFS or any other filesystem. That's up to you. ZFS or BTRFS are fine when indicated - and you need to know your stuff.

Cephs for clustering (hyperconverged) is very much a first class citizen. I generally only use EXT4 as a filesystem - keep it simple. XFS is lovely too, especially for reflinks if you need them.

(1) Wal and Cooch know how to run a farm (and so do I, in the UK!)