a DFU restore *should* remove the supervision
https://support.apple.com/en-gb/HT212787 on how to do a restore, which should also update the iPad at the same time
Whilst it is possible to change the ADE / DEP / Apple Configurator policies using a Mac and the profiles command, there's no such capability with iPad.
So, it looks like the only way to get iMazing to work is a full device wipe.