APFS Volume corrupted in Catalina and I can't reinstall OS

I appreciate that these things are posted in good faith. However I indicated (or at least implied) in my original that points 2, 3, 4 and 6 had already been addressed before I posted. (If I know I have over 220Gb free, but it's only showing 19Gb, why would I be looking to 'see used and available storage space'? It was this showing incorrectly that was the problem!)

Earlier conversations with Apple had also covered point 1, although yes, I didn't say that.

In one's reply one did describe trying your suggestion 5 and the outcome. One doesn't know how much more exactly one could have described that? :-(

After a further long conversation with Apple with one of the higher level specialists, the only course of action was the nuclear option of completely erasing the iMac, then reinstalling the OS and all my files. Fortunately I back up to Time Machine every other day, but could have done without the time and hassle factor. But at least I have a functioning Mac again.

P. Phillips wrote:

Start with

1 - Rebuild the Spotlight index on your Mac

2 - What is “Other” storage on a Mac, and how can I clean it out?

3 - Free up storage space on your Mac

4 - OmniDiskSweeper Safe to use

5 - How to delete Time Machine snapshots on your Mac

6 - See used and available storage space on your Mac

I have already cleared over 200Gb of space on the 1Tb drive, as I mentioned in my OP (I've been using Macs since 1993, so I have a good handle on the basics!)

The problem is that the Mac thinks it only has 19Gb of free space whereas it has over 200Gb.

When I tried deleting the Time Machine snapshot, Terminal returned the message unable to write to /var/db/sudo/ts/(my user ID): No space left on device