Try logging into another account (you may need to make one): does it still crash?
If you boot into Safe mode (hold down Shift Key at boot): does it still crash?
Have you tried running a free utility like Onyx to see if that stops the crashing?
https://www.titanium-software.fr/en/onyx.html
Another account - I always add one blank Admin account with every Mac system - just in case.
When I switched to that account of course I needed to login to Adobe (and my passwords etc are on the other SSD/account) - kind of a hassle so instead I decided to try booting from a copy of the system drive I just made a few days ago.
From there I was able to run LR (on my regular login) and import, and quit - as normal! I kind of knew that would happen however, since this bugger just popped up in the last day or so.
Back booting off the original drive and user account - I decided to give in another shot (maybe the 10th time would be the charm?). To my suprise - it did work!
The only thing I can think is when I switched back to the original SSD drive to boot from again, MacOS spun up hard (fans on full) so I looked at activity monitor and saw MRT was the culprit. Maybe something had found it's way into my system. Who knows? Whatever happened it now appears to have self resolved.
Total mystery. Glad I now have it cleaned up - so I can run my full bootable backup again!