Something is eating my SSD space

I have a 2014 MacBook Pro with a 256GB SSD, running Big Sur 11.7.10. I've moved about 50GB of items to an external hard drive, then moved the original items to Trash, then emptied the Trash. Disk Utility initially shows several GB free, then after a few minutes the free space is gone and the SSD is full. I've done this a half-dozen times.

I can't use my MacBook. Apps won't open. I'd show you a screenshot but it won't save screenshots.

The problem started this afternoon when I bought a topo map from Garmin and tried to download the map to my Garmin Oregon 700 GPS device, using Garmin Express. It downloaded 15% of the map (about 20MB). Then the download stalled because the SSD was full. I cleared 35GB from the SSD and restarted the download. It stalled at about 20% of the map, because the SSD was full.

Garmin support only works Monday-Friday. Garmin Express says that the GPS device software is up to date (and the Garmin Express is up to date).

I've disconnected the Garmin device and shut down all apps except Chrome and Disk Utility. I've restarted the MacBook.

I used Command-Shift-period to show hidden files. I've looked through my SSD but I can't find huge, new files.

All I can think to do is to backup the SSD to my Time Machine external hard drive, wipe the SSD, and restore from Time Machine. I did this years ago when the SSD was full, afterwards half the SSD was empty, and no files were missing. My Time Machine external hard drive has 2.28TB free.