There will be no performance gain even if you passthrough NVMe drives to that TrueNAS VM as you still need connect storage back to Hyper-V (iSCSI or SMB). You'll just add more latency.
Pvt-Snafu
Yes, that's possible and it will protect against a drive failure. But of course, you'll get the speed of the slower drive. Also, keep in mind that RAID is not a backup.
If you have an old PC, that could very well be a start. Otherwise, Dell Optiplex or Intel NUCs will be more powerful than Pi at the same price. Throw Proxmox on it and you have yourself a homeserver.
TrueNAS needs direct access to drives to ensure proper corruption detection and repair which is not possible with a hardware RAID controller. But if you're on ESXi, you could just deploy a Linux VM with Plex on a hardware RAID datastore.
Well, as others said, ideally, follow the 3-2-1 backup rule: https://www.backblaze.com/blog/the-3-2-1-backup-strategy/. An external drive plus some other cloud storage like Backblaze Personal or iDrive.
If you can still read it, I would start just manually copying the most important files one by one.
Most likely, the drive is damaged. there should be some local companies that do recovery. At least, it's worth asking.
And another vote for Proxmox just in case. For containers, just spin up another Linux VM.
As others said, Rclone and encrypt your data before sending. I wouldn't trust no matter what the cloud provider says.
I would go with Ubuntu Server. Anys OS will need maintenance. Alternatively, Proxmox if you're into VMs.
And that is awesome!
I'm using an old Synology for backups. But for a NAS, you could build a DIY machine and put TrueNAS on it.