No answers to my question, but for anyone else having this issue, I seem to have found a fix by deleting the saved password for that account in my Thunderbird, then going back to the new account and clicking check mail. A Gmail sign in box popped up and I entered my password (same one) again. All the emails for this second account loaded. I'm expecting glitches, but so far, it seems to be working.

Thanks for the feedback. This may be useful if someone reports having the same problem.

I didn't reply suggesting you to do that, even though I've suggested it in similar cases because (1) this being a new account setup, I assumed OAuth would have been automatically selected for authentication, and (2) it seemed strange to me that Thunderbird would silently fail to connect to the server without showing an error message…

Am I understanding it correctly that somehow you tried to set up the account without using OAuth2 to authenticate at first and that caused Thunderbird to silently fail to connect to the server?

EDIT: Reading the title of your post again, it would seem that you did successfully complete the OAuth2 authorisation process the first time, but Thunderbird silently failed for some reason, and you could solve the problem by removing the OAuth token (not the password) to repeat the authorisation process again and this time it worked…

Initially, I didn't know about the OAuth2 requirement, so I tried without it. When that didn't work, I went looking for answers and found out about it, so then I deleted everything and started over, using it. Everything seemed ok, but no new messages loaded. I found other suggestions, including clearing cookies, etc. and none worked—no messages loaded. Finally, after trying everything else, I completely deleted my password and tried without it...I got a popup to use my password to sign in, so I put it in and voila—everything uploaded immediately. It felt pretty random and extremely touchy, but once I had everything in place, I quit Thunderbird and restarted a couple of times to see if it would hold and so far, so good. If something goes wrong I'll be back. But if anyone else has the issue, I hope this is helpful.