If you do make a switch to Proxmox, then Proxmox Backup Server is where it’s at for backups. Its de-duplication feature is incredible. I backup all my Proxmox VMs/LXCs with it, as well as my non-Proxmox hosts (laptop, etc.), with proxmox-backup-client.

Personally, I’m using a few of those tiny Beelink PCs (a couple Mini S12 and an EQ12) with the N100 processor, as well as a couple larger rackmount PCs I built for situations where I needed to add an HBA or some other PCI-Ex device. I do recommend something like a Beelink before building, though - they run Proxmox fine, they’re inexpensive, efficient, quiet, and each one can run a handful of VMs.