Can not install Big Sur on 2019 27in iMac

Trying to install Big Sur onto my 2019 iMac, 27 inch 3.7 GHz.

I previously tried creating a boot drive on a USB memory stick to try to preserve access to 32bit apps. Memory stick meets all the requirements for this but couldn't get the boot drive to work.

Gave up on that and now just trying to install Big Sur, upgrading from Mojave.

The software begins the installation process and then just shuts down the computer. Just turns it off, doesn't restart at all. I understand that restarts are typically part of the upgrading process but after 15 minutes of nothing happening (no computer sounds at all) I pressed the power button on the back of the computer, white apple logo appeared for a few moments and then computer just clicked off. Tried again, same result.

Starting in recovery mode allowed me to re-choose the start up disk and computer would restart from the installed Mojave version.

Nothing I've done is any different from the "How to install Big Sur" page on Apple.