Update stuck at progress bar, presumably Monterey? Cannot boot into anything.

iMac 27 from 2021 running Big Sur without problems

System notification to update, Do It Later

Woke up in the morning and its still going.

Shows a black screen, apple logo and 30%ish progress bar and the message estimating time.

That progress bar did not move for over 24 hours

Started looking for ways to resolve this and have tried the following.

Unplugged and restarting the router

Unplugged and restarted the iMac (resetting SMC) and whatever else it does, back to estimating time

Both the above separately and at the same time.

Trying to log into Safe Mode, holding Shift key until Login screen, computer just reboots in a loop

Then read another question with similar description, (says Updating but nothing about Monterey), where it is suggested Safe Mode needs shift key until Apple Logo, tried that also and it just boots to estimating time.

Trying to boot to a different drive (Option key), progress bar to 60% then computer shuts down.

Trying to log into Update Installer, progress bar to 60% then computer reboots, progress bar back to 30% message estimating time

Normal restart, goes into Update Installer (I assume), progress bar back to 30% message estimating time

Tried to download the Install Big Sur to make a bootable USB, but the App Store won’t let me because it thinks I want to run it on my downloading machine, which is an old MacBook air 11, so cannot download that.

I tried to use an old USB drive with Install Sierra on it, just to get to the Disk Utility or the Boot Recovery but it insisted on an update, not sure to where, and then displayed Boot Recovery where nothing worked, I could see the app, Apple Logo, Menu Item on the top left but clicking did nothing, keyboard and wifi on right, but could not join wifi as password field always disabled, possibly due to keyboard not working. So that didn’t work either.

So now I have run out of ideas, I cannot boot into anything except a progress bar telling me it is estimating time, and I’m guessing it will show me that forever if I let it.

Does anyone have any suggestions, apart from the obvious, "take it to the store”.