macOS Catalina 200GB+ disappeared in several minutes

I don't know if any of this will help. I suspect there is an OS problem causing a mis-reporting of free space. If this is not the case, then the disk is so full that you are unable to delete files or even restart the computer. If you wish to try, then you can shut down the computer, wait 30 seconds, then restart. If none of this helps, then the only thing you can do is reinstall macOS but you would have to erase the startup volume first.

Where did all that free space go on my APFS disk?

How to check a Mac’s free hard drive space

How to free up storage space on your Mac - Apple Support

Reclaiming drive space by thinning Apple File System snapshot backups | Der Flounder

Working with macOS (Drive State) Snapshots

Time Machine Snapshots

Open the Terminal in the Utilities folder and enter or paste the appropriate command line. Press RETURN and enter your admin password when prompted. It will not be echoed. Press RETURN again.

     To turn them ON: sudo tmutil enable

     To turn them OFF: sudo tmutil disable

Note that turning them OFF will also delete all existing snapshots. For more about snapshots see: About Time Machine local snapshots.

How to Free Up Space on The Hard Drive

1. You can remove data from your Home folder except for the /Home/Library/ folder.
2. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.
3. Also, see Freeing space on your Mac OS X startup disk.
4. What is 'Other' and 'Purgeable' in About This Mac?
5. Files That Make Up the 'Other' Storage Category, and How to Remove Them
6. Free up storage space on your Mac.
7. See Where did my Disk Space go?.
8. Be sure to Empty the Trash to recover the space.
9. Replace the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.
10. Use OmniDiskSweeper or GrandPerspective to search your drive for large files and where they are located.

Give this a try:

Reinstall El Capitan or Later Without Erasing Drive

Please be sure you back up, if possible.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the side list.
4. Click on the First Aid button in Disk Utility's toolbar. Wait until the Done button activates, then click on it.
5. Quit Disk Utility and return to the Utility Menu.
6. Select Install OS X and click on the Continue button.