I don't think it's that file. The size of his Windows folder adds up fine when you put the content together. At this point, maybe a clean install would be a more sensible option? Just copy over our important docs/pics/vids/etc. onto another partition or drive and go on with the install.
@shhnedo I haven't researched it thoroughly, but could this kind of issue behave differently on Windows 11 Home compared to Windows 11 LTSC?
Thank you for your answer, and also thanks to everyone else who tried to help me with this issue. I will most likely format the C: partition and reinstall Windows 11 Home or the latest LTSC version, as it should be lighter and more suitable for my laptop.