Catalina: Login Screen Freezes After Password Entry iMac mid 2014 has been working fine for 5 years (well, sort of since I made the mistake of upgrading to Catalina).

As of today:


start the mac
select a user from login screen
enter password
A few seconds elapse and then the cursor changes to the spinning colour wheel


At this point the UI is frozen. I have tried leaving it in this state for 24 hours to see if it clears, it does not.
While in this state I can SSH into the mac and sudo kill -9 the login window daemon to get back to the user selection screen.

Tried so far:


Safe Mode login - same result
Safe Mode boot - same result
Reinstall MacOS - same result
SMC Reset - same result
Disk Utility fschk - no problems found
Macbook Pro stuck after I enter my password: I deleted csstore but no help.
My Mac gives me a black screen after logging into one account, but not another: Created a new user in the terminal via ssh. Same issue, can’t log in.


If I can I'd like to avoid having to backup data via scp, wiping the disk and reinstalling from scratch - I've got better things to do with my life.

I also have no intention of visiting a Mac Store only to be told my a smug child that he's going to wipe the disk and reinstall.

Someone out there must know what specific breakage Apple and snuck in via auto update the night before and how to undo the damage, surely?

Hardware:

    Hardware Overview:

      Model Name: iMac
      Model Identifier: iMac14,2
      Processor Name: Quad-Core Intel Core i7
      Processor Speed: 3.5 GHz
      Number of Processors: 1
      Total Number of Cores: 4
      L2 Cache (per Core): 256 KB
      L3 Cache: 8 MB
      Hyper-Threading Technology: Enabled
      Memory: 32 GB
      Boot ROM Version: 140.0.0.0.0
      SMC Version (system): 2.15f7
      Serial Number (system): DGKLG0SXF8JC
      Hardware UUID: 78775D43-2857-5894-88AC-36ED44B3DE84

Software:

    System Software Overview:

      System Version: macOS 10.15.2 (19C57)
      Kernel Version: Darwin 19.2.0
      Boot Volume: Macintosh HD
      Boot Mode: Normal
      Computer Name: iSpartacus2
      User Name: Richard Hodges (rhodges)
      Secure Virtual Memory: Enabled
      System Integrity Protection: Enabled
      Time since boot: 23:03
Not really a good answer. In the end I had to nuke the entire disk and reinstall from the recovery partition.

I'll stay with linux going forward.