This is a question probably better-suited for one of the Proxmox communities. But, I’ll give it a try.

Regarding your concerns about new SSDs and old VM configs: why not upgrade to PVE8 on the existing hardware? This would seem to mitigate your concerns about PVE8 restoring VMs from a PVE7 system. Still, I wouldn’t expect it to be a problem either way.

Not sure about your TrueNAS question. I wouldn’t expect any issues unless a PVE8 installs brings with it a kernel driver change that is relevant to hardware.

Finally, there are several config files that would be good to capture for backup. Proxmox itself doesn’t have a quick list, but this link has one that looks about right: hungred.com/…/list-of-proxmox-important-configura…