I have kind of a similar setup. Got started on my Proxmox journey late last year, and I'm a veritable noob with Linux as a whole. Migrated from a bunch of individual systems, mostly Raspberry Pi SoCs, to a very neat mini PC by Bosgame. The only difference is I run a separate Synology NAS that I mounted via SMB, so that all VMs can access it. I use Jellyfin as well. Granted, I'm running in a very basic RAID 0 with 2 8TB drives, but iirc it uses ext4. Since you have a full backup already, sounds like you've done half the work. Wipe the NTFS partition, and restore the backup to the newly created ext4 partition. I think that should work.
I'll try to answer any questions, here to help as I'm able.