I will suggest you just back up and start all over again. Mass migration of mail rarely works out if you do things like drag and drop in Thunderbird. It is at best slow and of questionable reliability.

My suggestion is go to "Google takeout" (such an American cultural name.) https://takeout.google.com/settings/takeout?pli=1

First thing, deselect everything using the button provided as they helpfully select everything Select the mail component files as mbox files and be sure to include all mail data. (this latter is the default).

I have not done this in some years, but the last time I did it I got an email about 24hours after I submitted the "takeout with a link to the relevant ZIP files containing the takeout mail. Keep the Zips under around 4GB unless your an apple person, apparently some apple and linux distributions have issues with Zips over 2Gb.

The Zip contained (or did for me) a collection of MBOX files one for each label and an all mail label as well I think.

Once you have the "takeout" stuff

Create a sub folder in Thunderbird "Local Folders to hold your GMail. Open the troubleshooting information on the help and click the open button in profile in Application basics. Shutdown Thunderbird in the file manager window that opened select and open the mail folder, then the "local folders" folder Now open the folder you created in Thunderbird to hold your gmail. Place the mbox files from the GMail zip into this folder. Rename each of the mbox files so they have no file extension. Windows loves fie extensions and they are hidden be default, so you will need to make them visible before you can start removing them. Files such as inbox.mbox need to end up a inbox. without the mbox. Once this occurs, Windows will stop telling you they are type MBOX. Restart Thunderbird. Access the folder you create for your gmail. Everything should be there. Initial access to each folder will be a little slow as the file will need to be scanned to build the list of emails, but it should be in the seconds to display, not the minutes.

Once the gmail is in local folders I suggest you look at the archive settings and functions. Very simply way to get mail into years and because it is operating on local files fast.

I will suggest you just back up and start all over again. Mass migration of mail rarely works out if you do things like drag and drop in Thunderbird. It is at best slow and of questionable reliability. My suggestion is go to "Google takeout" (such an American cultural name.) https://takeout.google.com/settings/takeout?pli=1 First thing, deselect everything using the button provided as they helpfully select everything Select the mail component files as mbox files and be sure to include all mail data. (this latter is the default). I have not done this in some years, but the last time I did it I got an email about 24hours after I submitted the "takeout with a link to the relevant ZIP files containing the takeout mail. Keep the Zips under around 4GB unless your an apple person, apparently some apple and linux distributions have issues with Zips over 2Gb. The Zip contained (or did for me) a collection of MBOX files one for each label and an all mail label as well I think. Once you have the "takeout" stuff Create a sub folder in Thunderbird "Local Folders to hold your GMail. Open the troubleshooting information on the help and click the open button in profile in Application basics. Shutdown Thunderbird in the file manager window that opened select and open the mail folder, then the "local folders" folder Now open the folder you created in Thunderbird to hold your gmail. Place the mbox files from the GMail zip into this folder. Rename each of the mbox files so they have no file extension. Windows loves fie extensions and they are hidden be default, so you will need to make them visible before you can start removing them. Files such as inbox.mbox need to end up a inbox. without the mbox. Once this occurs, Windows will stop telling you they are type MBOX. Restart Thunderbird. Access the folder you create for your gmail. Everything should be there. Initial access to each folder will be a little slow as the file will need to be scanned to build the list of emails, but it should be in the seconds to display, not the minutes. Once the gmail is in local folders I suggest you look at the archive settings and functions. Very simply way to get mail into years and because it is operating on local files fast.

I appreciate it. I did a TakeOut archive already just in case I blew things up. :) I've done this a bazillion times in Outlook, but I like more about ThunderBird for the most part. Outlook is ok, but Thunderbird just seems to work the way I think. I think that doing it by the 6 month range is working. Never again will I let my emails stack up this much. This is terrible.

I've done this a bazillion times in Outlook, but I like more about ThunderBird for the most part. Outlook is ok, but Thunderbird just seems to work the way I think. I think that doing it by the 6 month range is working. Never again will I let my emails stack up this much. This is terrible.

Do note that Gmail has bandwidth limits. See https://support.google.com/a/answer/1071518?hl=en