> Once that gets filled up, it’s pretty much impossible to understand where the giant block of memory is.

Your friend is called ncdu and can be used as follows:

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

The exclude for Volumes is necessary because otherwise ncdu ends up in an infinite loop - "/Volumes/Macintosh\ HD/Volumes/" can be repeated ad nauseam and ncdu's -x flag doesn't catch that for whatever reason.