Catalina freeze on boot after upgrade

After updating my primary computer with the recent Catalina upgrade, then on boot the progress bar goes to 100% and stays there.

Steps - once:

1. Upgrade to current version of Catalina
2. On completion, Catalina warns me that the current driver for my Kensington Expert Mouse won't work in future OSes so I go to the Kensington site and get the latest drivers and install them
3. System asks me to authorize the new extension and I do so

Steps - repeatedly:

1. I eventually shut down and the next day I boot up
2. I have FileVault enabled so it gives me a choice of which user
3. I pick a user
4. I enter my password
5. Progress bar appears, slows down at about the 2/3 mark, and crawls to 100% and sits there.

I've tried launching in safe mode (space bar) multiple times with no success.

I've tried disabling FileVault. Now on boot the progress bar appears immediately, slows down at about the 2/3 mark, and crawls to 100% and sits there.

And I still can't get it to launch in safe mode.

I've tried letting it sit for a few hours and it continues to sit there. That said, I haven't tried leaving it on, say, overnight. If I were to wake up to a computer that had turned itself off, I would know it was doing something, completed it, and timed out because I wasn't there to choose a user and enter my password. But it wouldn't be practical for me to have to

I've tried launching in recovery mode (command + R) multiple times with no success, although it is a non-Apple keyboard and Apple tech notes suggest that may be an issue).

I can boot into target disk mode (T). I can connect to the problem computer using target disk mode from a 2nd Mac, which is on Mojave.

I've seen a post suggesting this freeze is common after upgrading Catalina from one version to the next. I've seen suggestions that local snapshots may be the culprit.

So, in terminal, I tried the following:

tmutil listlocalsnapshots /

when executed on my 2nd Mac show the 2nd Mac's local snapshots.

tmutil listlocalsnapshots [my problem computer]

don't list any local snapshots. Where would they be?

I've seen a post suggesting that third party extensions can also be the culprit (which is why I'd love to be able to boot into safe mode).

But if I go to [the Mojave Mac]/[the Catalina Mac in target disk mode]/System/Library/Extensions I don't see the Kensington extension that I added. The extensions that were updated in 2020 are all Apple extensions, not third party ones.

Any ideas on how to proceed via target disk mode to:

1) Find and clean out any local snapshots on the Catalina computer

2) Force the Catalina computer to re-boot in safe mode

3) Find the Kensington extension on the Catalina computer in order to delete it in case it's the issue

?