Just to update on what I decided to do.
Have gone with Promox on the new host, ZFS Raid, and have already migrated some of the smaller VMs that didn’t need additional disk space above a single OS drive while I wait for more storage to arrive.
For backup with Hyper-V I am using a combination of Veeam for OS drive images, UrBackup for a couple of VMs for image/file backups (as I had run out of free licences for Veeam), then wherever I wanted file backups of the VMs and also some other machines, I use Duplicati. All backups go to a dedicated local server (which is running Veeam and UrBackup), and the file level backups are also replicated to Wasabi S3 for offsite.
So for the new host the issue is the image backups being as neither Veeam or UrBackup helps, so I have gone for using the PVE backup option (not the Proxmox backup server which I may look at another time), sending the files to a share on my backup server, and then the file level backups remain with Duplicati.
I will see how this goes but it means I can drop Veeam once those backup’s retentions expire, and probably UrBackup as well, though I do hope it gets Proxmox support in the near future - which I too am happy to help test.
Thanks everyone for your input.