Converting the physical PC to a Hyper-V virtual machine - Just download the Disk2VHD application from Microsoft and run it.
For any existing KVM/QEMU installation, the conversion of KVM's VHDX format into Hyper-V can be done using the qemu-img command e.g.
qemu-img convert -f qcow2 something.qcow2 -O vhdx something.vhdx.
And in case you are trying to do this conversion on a Windows machine instead of the usual scenario which is likely a Linux machine, the windows QEMU tool can also be found via this URL https://cloudbase.it/qemu-img-windows/
Converting any Hyper-V over to KVM was also easy enough.
https://www.servethehome.com/converting-a-hyper-v-vhdx-for-use-with-kvm-or-proxmox-ve/
iMac (OSX) - Whilst the WIndows installation did run on the installed QEMU, the Virtio drivers from Fedora did not work as expected.
Acer (Ubuntu) - Already running another Windows VM which leaves only the problem of getting the virtualised VHDX file to boot.
Lenovo (Hyper-V)
This was easy enough, simply enabling the Windows feature (only available on Pro versions and not Home versions).
https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/quick-start/enable-hyper-v
After running 2 VMs on the same physical machine for two months, my personel conclusion is not to try it with anything below 16GB RAM and 512GB hard drive.
I am trying a compromise, after I had first upgraded from 8GB to 16GB RAM, only to find that I had also run out of storage space because the first VM alone took up >100GB, of using an external hard drive to store the VMs. Will update once I figure out whether there are any performance hits with this approach.