Can’t get past Login screen

A Mac Pro from 2019 does contain a T2 Chip ( Secure Enclave )

Mac computers with the Apple T2 Security Chip

If what is feared, the Firmware using in the Secure Enclave may has become corrupted or damaged

Which may account for the loss of Admin Privileges on the Original Admin account and the Secondary Admin account

In which case a Revive or Restore in DFU Mode maybe needed

Secure Enclave

The Secure Enclave is equipped with a dedicated secure nonvolatile storage device. The secure nonvolatile storage is connected to the Secure Enclave using a dedicated I2C bus, so that it can only be accessed by the Secure Enclave. All user data encryption keys are rooted in entropy stored in the Secure Enclave nonvolatile storage.

Counter lockboxes hold the entropy needed to unlock passcode-protected user data. To access the user data, the paired Secure Enclave must derive the correct passcode entropy value from the userʼs passcode and the Secure Enclaveʼs UID. The user’s passcode can’t be learned using unlock attempts sent from a source other than the paired Secure Enclave. If the passcode attempt limit is exceeded (for example, 10 attempts on iPhone), the passcode-protected data is erased completely by the Secure Storage Component.

SBJax wrote:

So, I'm left with the nuclear option and a restore from a 6 month-old Time Machine. My concern is if this will still not resolve the underlying issue IF it is, in fact, a firmware corruption.

A Nuclear Option with a T2 Chip machine ?

Proceed with an abundance of " Caution "

Part 1 of 2

This method will WIPE ALL Data and there is No DATA Recovery - Period.

>> Only works on Intel Based Apple Computer <<

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can’t be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

Extra Special Notation regarding the Touch ID equipped Apple Computer.

 About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

Part 2 of 2

Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

Choose the Upper Most Drive ( not the volumes indented and list below ).

The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map.

Once that is done >> backup out of Disk Utilities and choose install macOS. 

Follow the prompts and it may automatically reboot several time. 

Upon a final reboot - Setup Assist will present with the newer version of macOS.

You can use Setup Asset and point it to the Time Machine Backup Drive. This is a One Time offer. Thereafter you will need to use Migration Assist 

If the guest account works then most likely there are extensions installed on the regular accounts that are incompatible with Tahoe. Safe Mode is the answer to that, what do you mean by ‘it never seems to work’?

Between the advise from my colleague @ neuroanatomist and self 🦉-53

I too am " wits end "

May consider, Make an Appointment at the Apple Genius Bar or call 1 800 MyApple and have them evaluate the computer

Other location Call Centres 

 Contact Apple for support and service

I would suggest leaving your pride at home and take the machine into the local AARP