have you exceeded you gmail daily data limit? Have you disabled any email scanning in anti virus that would certainly slow things, and may crash them given the volume?

Did you do this in small batches or just expect a program that is to manage emails to upload 30 thousands of them in one go. That usually fails at some point. Batches under a thousand almost always work.

I didn't realize there was a gmail daily limit. I did it in mostly batches of 3-5K. Sometimes in batches of only hundreds. What should I do at this point? I have all the emails where I want them in folders in Thunderbird. How do I get my new gmail account to show those same emails in the same folders? Do I have to start over and do smaller batches? I was basically just dragging and dropping from an .mbox (gmail1 export) folder that I put inside Thunderbird.

Do not drag and drop. Use right-click - Copy To instead. Copy messages in smaller chunks. Only delete messages in the source folder once they have been successfully copied to the server.

My recommendation would be close Thunderbird and leave it for 24 hours before trying again.

Then see if the sync will resume. It might or might not. upload the mails, but it should sync the two folders in one form or another.