Also note that there is a simpler approach if you don't need an "all-in-one" ISO image: driver disks.
You can build driver disks using https://github.com/xcp-ng/driver-disks and load them during the installation.
You can also ask us to provide driver disks for alternate drivers for which we haven't built them yet. We make them available on https://mirrors.xcp-ng.org/isos/drivers/8.x/...
That's also something we need to document in the official docs. It's mentioned in the release notes for 8.3 but not in other sections of the documentation.