For the extra cost, I don't think an NVMe drive would be worthwhile in this scenario. If all you need is reasonably quick storage expansion for stuff like games, photos and other files (as opposed to needing the space for something more intensive like processing a lot of high definition raw video files), getting a SATA SSD and putting it in the free internal slot is the smartest solution. You can pick up decent 1TB SSDs for $90-$110.