Well, it took a couple of hours but it's working.
As mentioned in the previous message, when I try to install FormIt360Converter.msi from the original image, it nags about a transform it can't find. So I removed Revit 2023, created a new deployment for Revit 2023 and installed it. The installation took a loooong time, so I checked the install log. Noticed it had already installed everything but FormIt360Converter.msi failed. Apparently, this kept the installer running indefinitely.
With a new image, I had a new FormIt360Converter.msi which is slightly newer. And when I try to install that one, it nags about an older version it cannot remove. Searched the registry for FormIt360Converter.msi and it turned up under [HKEY_CURRENT_USER\Software\Microsoft\Installer\Products\FF5EDC4656A73384995073E49A646CF8]. Renamed that key to .old and tried another install. Which it did without nagging. Ran Revit and ... problem solved.