First, IMAP was not designed for massive uploads. In doing it, you may have difficulties and, even when messages are uploaded, you need to spot check to verify that the full messages are there. My suggestion is to do small groups at a time and to create a filter to manage this. Having a manual filter automatically slows the process and that helps ensure the upload is successful. I suggest treating each folder separately.

Thanks for your reply. Unfortunately, it's now not even allowing ONE folder to be copied. I'm getting a Tbird pop-up error message that says "mail server contains excess white space." I'm not a tech person. I have no idea what this means.

You are not a tech person, but you are attempting a huge backup using a tool that is not designed for the purpose. Either you need to get technical or well, I really don't have another option.

My guess is Google have suspended further uploads for 24 hours as you have expended your allocated daily bandwidth. Like everything you pay for (even with your personal data) there are limits and google do publish theirs. They are generous for getting and sending mail and backing up files to google drive. Not so much for trying to use gmail as a backup drive.