Mojave to Big Sur update impasse

Sorry to be boring, update problems can’t be everyone’s favourite topic, but I’d really appreciate some advice.

iMac 21.5" 2017, Fusion drive, running Mojave 10.14.6. My update to Big Sur (following instructions in Take Control) seemed to be going fine until after about 40 minutes a progress bar froze before it got to the end, and stayed that way ever after. (This was yesterday.)

Restarting just produces an apparently stationary progress bar again.

Shift-start for Safe Mode produces a startup chime but not Safe Mode, instead it eventually goes into the Big Sur initial setup process but crawlingly slow, there’s clearly something very wrong.

Restarting with Command-R produces a Recovery Mode that seems to run at normal speed and I selected Disk Utility, but it found no errors in Macintosh HD or HD - Data. The drive has been reformatted to APFS.

So I decided to go for a clean install. I have a bootable copy of the original system on an external hard drive, including the Big Sur installation app, so I launched Disk Utility from there to erase the internal drive. However, selecting the top-level Fusion Drive icon greys out the Erase option. I could erase ‘Container disk2’ which contains two drives, Macintosh HD - Data and Update. Is that the right thing to do?

And if not, where do I go from here, please?!

I eventually solved this problem although it has left the folder structure in a bit of a mess. There’s a brief description in my later posting, ‘Fusion Drive - which volume to erase?’