Try a new profile (https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles).

I can't even create a new profile (won't save changes). I tried with Thunderbird opened & closed.

I had the same problem and resolved the issue in the following way: 1. I first moved all my messages back to draft, and sent one by one and found one gmail account not sending, and this caused all other email accounts to just also sit there. 2. I check the error logs and found the gmail account settings complained that it could not find the smtp configuration for the server. 3. I went into config editor and found the smtp server id was linked to another smtp configuration. I changed it back to the right account id number (after i could not get it to do it via the normal account settings manager, must be a bug there too) 4. I had to restart thunderbird several times before settings were retained properly. 6. It then failed with auth, and found the server had reset to normal password sending, changed that back to Oauth2 and restarted TB. 5. I went back into the draft and now re-opened every message for that server and it then sent out the mail successfully.

Anti-virus software?

Only have Windows Defender antivirus

I can't even create a new profile (won't save changes). I tried with Thunderbird opened & closed.

Without uninstalling, try reinstalling TB from https://www.thunderbird.net/en-US/thunderbird/all//

Start Windows in safe mode.

Start Thunderbird in troubleshooting mode (Help menu).

Test.

Let is know how it went.