100GB+ of hidden space I can't delete or purge

Solution 1:

Try OmniDiskSweeper which shows in a column-view:

Largest folders will sort to the top.

I often will run it via

sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper 2>/dev/null 

Also: make sure that it gets “Full Disk Access” as shown here:

Solution 2:

Expanding on modlin's answer, when using Ncdu:

ncdu /

on macOS, you can avoid the firmlink problem by excluding /System/Volumes/Data:

ncdu --exclude /System/Volumes/Data -x /

Solution 3:

Also try 'Boot Camp Assistant', click Continue and then wait.

You are just use it to trigger some cleanup action, not actually installing Windows.