I like the idea of exposing a new LUN to the VMware guest via iSCSI. That would get you around the 2TB vdi limitation on your xcp-ng cluster, and let you just detach from the old and reattach on the new all via iSCSI.

The part that feels a bit risky is setting up the Windows RAID mirror with an existing drive with data. I don't work with Windows Server all that often, so maybe it's easy and knows not to risk the existing data.
I would probably replace that step with some Robocopy tasks.