Well, file system discussion is similar to OS discussion: There are systems coming from the high end and went down, and systems basing on small computers and grew up with their facilities. ZFS is one of the former - brought down especially by (Free)BSD - and md with ext4 one of the latter. Btrfs is a later development inbetween, trying to bring the best of ZFS down to smaller hardware.
During the past 10 years I used all three FS and can tell, that FreeBSD implementation of ZFS was my favorite - until FreeNAS (today TrueNAS) moved from FreeBSD to debian linux (sic!) and the branch NAS4Free (today XigmaNAS) still does not really support Multi-Gb LAN. And btrfs users (Synology and Rockstor) still warn about using it with RAID5 or 6 configurations without an UPS...
So when I got the hardware below, and saw, that latest proxmox rely on ZFS, too, I decided to go back to OMV, of which I used versions 2 and 3 already, and use the proxmox 8 kernel with it, that supports ZFS natively, rather than as the addon in former versions.
It looks to be a good decision. Even if ZFS not seems to allow reducing the pool size yet like md or btrfs. But this is just a need to make sure about, what I really want. I.e. if I really want to accept possible break-down of my whole 6-HDD-RAID5/ZFS-1, if I need to replace one of the disks, or if I rather setup a RAID6/ZFS-2, which still allows a second disk to give up while resyncing the RAID...
BTW: That was, what I meant, when I wrote, that I would not use RAID5/ZFS-1 with eight HDDs no more.