Check out Proxmox + https://tteck.github.io/Proxmox/ + lxc container snapshots on the NAS and set up Proxmox backup server on the Pi. I find such a setup to be "all benefit, no giving up anything", contrary to NixOS.
Yeah, I love that stack. I then rsync those backups to backblaze.
No issues, single machine, no fucking around with stuff like kubernetes (and I also got rid of docker, so that annoyance is also gone)