It’s best to just let it finish as any other method you might try you could end up with invalid sync state, which will cause issues long term.
Not sure why it’s so slow though. I have thousands of items too and it takes less than one hour to do a full sync with Nextcloud. Maybe it’s something with Dropbox that makes it throttle the connections.
Is the Android on your local wifi? Using 3g data for a full transfer would be slow. If you have your data on an older SD card, they sometimes can be super slow. Did you enable E2EE (encrpytion) that seriously slows down a phone. Although the sync should be faster than 2 days. I can see hours but days?
I’d highly recommend leaving it go till it’s finished.