You can use regedit to make the LTSC IoT installer think you're already on an LTSC IoT build so it just installs without doing a clean install.
I first learnt about it from this Youtube video but they only show how to get the base LTSC version and not IoT which will get updates until 2032.
Here's the values I used.
"CurrentBuild"="19044"
"CurrentBuildNumber"="19044"
"EditionID"="IoTEnterpriseS"
"ProductName"="Windows 10 IoT Enterprise LTSC 2021"
"ReleaseId"="2009"
"DisplayVersion"="21H2"
I have them in a registry script along with txt guide I've been sending to my friends. Not sure if I can directly post them here however.