I've got one of the 3 month Game Pass codes. If I use that, will I still be able to get the introductory price of £3.99? Or if I use the code will I lose out on the introductory offer?
Depending on the pass, may simply be able to apply it after you use the introductory price to simply extent the period. There are some pass's though which you can only use with a brand new account for first time which can catch you out so just double check.

The problem is I don't have the space to move everything onto another drive and it will take a week to re download everything with my connection.
I've done it before with FAT32, it takes just a few minutes. But haven't tried it with exFAT before. Mind you, this wasn't your problem. The NTFS block size issue can't be fixed with the convert command.
meant to play that but never got round to it