My iMac freezes at "Less than a minute remaining..." and I'm not sure what to do to resolve the issue. I've forced a shut down twice and keep getting to the same "Less than a minute..." freeze

Suggestions? Yes. Let it run overnight.

Some of these time predictions can occasionally be… off. Longest I’ve seen for an update or upgrade “surprise” was three days.

On the possibility that there’s something else going on here, how current are your backups?

If this iMac 21.5” model is the 8 GB / hard disk configuration, that configuration is not known for its performance.

Depending on several variables... I've sen that last minute take nearly an hour, how long are you waiting?

Try reinstalling it in Safe Mode...

Safe Boot, (holding Shift key down at startup).