Uploading to gmail from local folders

Thank you in advance for any help/advice... I recently noticed that for a very long time I've been archiving all of my emails thinking they're going in the trash. It's resulted in thousands of email that went into the "all mail" folder of gmail. Long story short, I made a 2 folders in the "local folders" area of Thunderbird for my inbox (emails I want to keep) and my outbox (also emails I want to keep) and I successfully downloaded my entire inbox and outbox to those folders. Then I deleted all the contents on the gmail server with the intention of uploading my inbox and outbox back onto the server, essentially finagling a way of deleting all of the emails I didn't want that had been placed in the "all mail" folder—probably 20,000 emails. The problem I have now is that when I try to copy the contents of my local folders (inbox and outbox) to the inbox/outbox on the gmail server, it uploads a single email every minute or so and then tries to download the emails once they're uploaded. There are 34,000 emails in total and I've left this going for a day and only about 200 emails have been transferred. My internet speed is also really fast, so it's not that. And the downloading of these emails to my local folders went really fast. I have no idea why or how to do this more effectively. To sum it all up, I need a better way to transfer my emails from my local folders back onto the gmail server. (All of this initially began with the intention to conserve server space, as I'm convinced that we all need to be more mindful of how much space we're using up on the cloud...) Please respond if you understand Thunderbird or gmail better than I do and can shed some light.