Catalina install fail, will NOT go into Safe Mode

I have a 2018 27" iMac that's always performed flawlessly. That is, until trying to install Catalina. Like another issue in the forums marked as RESOLVED, it hangs at just before the bar finishes. This machine rebooted itself at least four times, and I've since rebooted it another four or five times to get out of this screen.

So I followed the advice in that article which said to download the update manually, then reboot into SAFE Mode.

Unfortunately, it's totally stuck on this screen. I reboot, hold the shift key, and the bar goes back about a quarter inch from the right, and works its way back to showing 1/8" to go. Period. It will not leave this screen.

It's starting to feel like this install bricked the machine. Grrrrrrr!

Please help! Thank you!