This all could have been done in 3 simple steps, i am creating a video as we speak, no need for any autounatend.xml stuff.
That upgrade path should be possible, maybe it needs a license switch to normal Enterprise 2021 LTSC and then upgrading to Enterprise 2024 LTSC and next switching back to IoT Enterprise 2024 LTSC by Key. Or try this tool to perform the upgrade directly: UpgradeMatrix.xml FiX Tool (Very Beta)