Do you have enough storage on your Google Drive for all those emails? The free 15GB allocation may not cut it.

There is an add-on for TB 78, 91 and possibly later designed for this purpose:

https://support.mozilla.org/en-US/questions/1331879

https://github.com/smitgd/addon-copyfolder-tb-esr68/releases/tag/v2.2.2

You can install an older TB to a separate folder and run it on its own profile, if you already have TB 102. Or, install the old version on a separate PC. Otherwise, uploading in small portions is the only option. Ask yourself if you really need those messages available on the IMAP server.

The google drive should have enough storage.

@sfhowes that is helpful. I cannot seem to find the add-on in TB looking for 'add-ons' via this link https://addons.thunderbird.net/en-US/thunderbird/search/?q=copyfolder&platform=all&appver=any&page= Do I need to install it via github? If so, could you walk through how to do that? Thanks so much.

Merry Christmas and happy holidays! Thanks again for the help here!

Drag the link to the xpi file on github and drop it onto the TB Add-ons Manager. Or, download the xpi file, and click the gear icon in the Add-ons Manager, 'Install add-on from file'. Once installed, right-click a Local Folders subfolder, (Add-on) Copy Folder To, select the target folder. I tested this in TB 91, and it seems to work. Keep in mind the gmail bandwidth limits.

It that doesn't work, I've used filters to move large quanties of mail. Just use a filter rule that would match all messages. Or most messages, and do a little clean up later.

It essentially moves one message at a time, and therefore shouldn't run into resource and imap problems that result from selecting and moving hundreds of messages at a time.