Mail.app constantly re-downloading hundreds of IMAP messages

For a couple years now, Mail.app (on a 2012 MBP running Catalina) has frequently shown mysterious sending activity that doesn't correspond to any recent messages — getting stuck "Sending 6 of 7" or "Sending 18 of 19," but always stuck on the second-to-last message, even though no group message has been sent. I've posted here a couple times about that and haven't been able to find a solution.

But now it's gotten worse with a new problem: Every day Mail.app now wants to re-download/re-sync hundreds to thousands of emails. This computer lives at a remote ranch that can only get online via satellite, which has a 10GB/mo limit — and all that bandwidth is being used up in a matter of days because Mail won't stop doing this:

If Mail is left running, it will do this dozens of times a day — often removing hundreds of emails from the Inbox and re-downloading them.

Any suggestions on how to track down this problem?

Deleting and recreating the account isn't an option because, as mentioned above, there's not enough bandwidth to re-download 25 years of mail — and there's no guarantee this would fix the problem. I'd like to figure out the cause.

(I have a sample from Activity Monitor, but I haven't uploaded it anywhere because the last time I did that, I found out it included OAUTH data.)