Hidden files can’t be deleted after Big Surprise downgrade

I had to downgrade back to Catalina from Big Sur. I formatted my HD and installed Catalina from a bootable flash drive. After installation I only had about 20 GB of free space on my HD, more than 200 GB was taken up as hidden ‘other’ files. I tried formatting the HD again and installing Catalina again, same problem, only now I only have 9 GB left…I have tried all the usual ways to find/delete these hidden files to no avail, including scripts in Terminal, various disk sweeping apps, etc.