I have been migrating VMs from Xen on Debian 11/12 using the python2 script this past week and it worked best if I had ensured the VMs were booting on Debian using pygrub as the bootloader rather than the specified kernel from the Dom0.
Then all I had to do after running the script was go into Xen Orchestra and change the "PV args" for the VM under "Advanced" to "root=/dev/xvda ro" (instead of "root=/dev/xvda2 ro" from the imported config), edit the "Network" config to match the MAC addresses used by the VM before, and mount the tools disk.
The VM will then boot and I can edit the /etc/fstab file to use xvda and xvdb for the boot disk and swap respectively (instead of xvda2 and xvda1 that Debian had been using).
I edited my VM's /etc/network/interfaces (because we have standardised on having the internal network as the first ethernet port rather than the second), then mounted the tools and ran 'sudo /mnt/Linux/install.sh' before rebooting. It is a pretty quick process once you've done a dozen or so!!
I tried updating the script to use python3 but as I am not a python developer I decided it was safer to build python2.7 from sources to run on Debian 12, since unfortunately python 2 is no longer available from packages on Debian 12.
Thanks for sharing your experience here, which I thought I would just add to as it may help others migrating to xcp-ng over coming months/years. All I can say is, "Do it!". The sooner the better!
Once I have all the VM migrated in PV mode, I will then work on moving to HVM.