Catch-22: FileVault Encryption Complete, Can't Use Time Machine or Update OSX Because HD is Being Encrypted

I'm confronting an OSX bug. Here are the symptoms (not necessarily in order discovered):

Hard Drive is encrypted with FileVault. I have verified through System Preferences and Terminal Mode that encryption is complete. For example, diskutil apfs list shows FileVault: Yes (Unlocked) for the Macintosh HD (FileVault: No for the Preboot, Recovery, of VM volumes).

Under System Preferences/Security & Privacy/FileVault, the option to Turn Off FileVault is greyed.

Time Machine will not complete backups "because FileVault was encrypting or decrypting a disk."

I have tried turning off Time Machine, deleting the com.apple.TimeMachine.plist file, then reselecting the external backup drives.

Shut down does not complete, even after waiting days. I have to use the power button to shut down. Shutdown will not complete even when in Safe Mode (but will complete from Recovery Mode).

I have no apps that Open at Login.

Both MacOS Mojave Security Update 2021-005 10.14.6 and the macOS Monterey update will not install. Attempting to do so hangs up the computer, and when I use the power button to shut down and start up again I have to use Startup Manager to remind it where the operating system is (and use System Preferences to reassign the Hard Drive as the startup disk so I don't have to do this every time).

These problems persist even if I clear the SMC, the NVRAM, start up in Safe Mode.

I have used Option-D to confirm I have no hardware problems.

When I disconnect everything but the monitor, mouse, keyboard, and internet; clear the SMC and NVRAM/PRAM; start up in recovery mode; mount and repair/confirm there are no problems with the hard drive; and then try to reinstall the operating system, I get the error: “You may not install to this volume because it is currently being encrypted.”

Reading up on these issues, I found reports that older apps could cause some of these symptoms (and in particular some LogiTech and some older .kext files that I too had on my machine). I have eliminated older apps I no longer use, including all LogiTech apps, along with their related files. I used a third party app (AvastCleanup) to eliminate these programs.

My problems seem to have started around the time I changed my FileVault password (thinking I would make it more secure by picking something harder to guess). More recently my Hard Drive was getting close to full. To free up room I have moved my Photos library to a USB drive.

Because the System size appeared unusually large, I have done some things to clear System Data storage (e.g., deleting Cache and Log files).

Based on one web posting, I used Terminal Mode to try to delete some of the Time Machine local snapshots, using "tmutil listlocalsnapshotdates" and "tmutil deletelocalsnapshots (paste from list)". There may be some problem with these files. Even after saying a file was deleted, it shows up again when I use the list command.

After these steps to reduce the System Data storage, it has come down to 567.6 GB (more than half of the storage in use).

I have a 2TB hard drive. Recovery mode Disk Utility for the HD indicates that 1.98 TG of this is used, 11.73 GB are in Other Volumes, and only 133.11 GB are free, with Zero KB purgeable. It also shows for the macOS Base System that 1.28 GM is used and 732.8 MB are free. After I reboot (so I'm no longer in Recovery Mode), "Get Info" for the HD shows 1.98 TB used with 1.2 TB available (1.07 TB purgeable). The "About This Mac" Storage tab also shows 1.2 TB available with the System using 567.6 GB. About This Mac/System Report/Hardware/Storage shows only 133.04 GB available. When I first started looking at the size of the System Data files, it appeared to be fluctuating, but after the actions described above it seems to be giving the same results each time I check.

One other odd anomaly was that a couple of weeks ago the Spotify app began starting up at login even though it was not listed as a Login Item. I went ahead and deleted the app as a precaution, but this did not solve the problem or make any difference. I have no Login Items under System Preferences/Users & Groups/my username.

I tried going into Terminal mode and using sudo fdesetup disable following the advice in Cannot Turn Off FileVault - Apple Community and got the FileVault was not disabled (-69595) response, even though I have confirmed Secure token is ENABLED for my username (and the other username in my system). The conclusion at the end of this thread is to wipe the drive and start over.

There are multiple posts across many sites by people having some or all of these symptoms and issues. This appears to be a long-standing and unresolved bug. These other discussions all appear old enough that they have some variation of the discussions.apple.com statement: "Looks like no one's replied in a while. To start the conversation again, simply ask a new question." Hence I am asking a new question.

Anyone have any suggestions?