The Xen / XCP-ng equivalent for VMXNET are Xen PV device drivers which either come from Citrix Xenserver, XCP-ng or the Xenserver project itself. Check the Wiki. It describes the installation pretty well: https://xcp-ng.org/docs/guests.html#windows As for 3D acceleration within the VM there is hardly a comparable option unless you do some PCIe pass through of a GPU.

Just scrapped the previous XOSTOR install. I'm going to "trial" XOA + XOSTOR 1.0

Since you're changing all of the underlying hardware that the guest has, Microsoft is going to force you to reactive. You could look at setting up a licensing server to manage these systems and then feed them that information so they can validate and then license. This isn't an issue presented by XCP-ng (or literally any other hypervisor), its a Microsoft restriction to ensure you aren't stealing their software.