Brief thoughts:
I use ProxMox and PBS personally and with clients. Clients have local PBS, that PBS then connects over a VPN to my PBS and syncs the delta every night. (Hint hint: make a small PC and send it to @marcoarment padding it with electronics you don’t need and he can store it next to the water in his closet.
I run PBS with ZFS based data stores and I’m able to use a mirrored config doing so, currently using two 4Tb disks. Since it is only two disks if both fail I’m hosed but if one fails and I catch it fast enough it’s easy to replace. I don’t actually need the 4Tb of space, between my backups and my client backups PBS is still saying it will take a year at the current rate to fill up the disk, and that’s presuming I don’t clean up old backups after 6 months.
As for replacing the Synology the Unifi gear works well, if you want something with more options TrueNAS Scale is another choice.


