So the subnet mask is got from the device handing out dhcp. Not 100% sure but on my android the subnet mash for wireguard is as /24 set on the device and also matching in the wireguard settings in opnsense. Opnsense is very very powerful, I would watch a few videos on YouTube about subneting, wireguard routing & dhcp. Its gonna be quite the learning curve (or could be)
As to why everything has stopped working who knows…
I run snapraid and mergerfs, as the nas storage. Not much changes on my NAS and the stuff I really care about like my pictures and videos are on a small ZFS pool. Both are directly on proxmox, meaning I can just plug them in to another Linux machine and research if it all goes sideways. Its all shared from the host via SMB NFS or for jellyfin and immicher its a moint point for the container