Buying a 2019 iMac - I am getting the 256GB SSD upgrade. I cost of going from 256 -> 512GB is £160. I can buy a thunderbolt 3 SSD on amazon for £90 for 512GB, and £160 for 1TB. One big thing I want to do with this iMac is a lot of photo editing utilising Photos app (which I understand can all be stored externally) and a fair amount of video editing. I don't mind spending the £160 to go 512GB internal if it is far superior, but the way I see it I may get more bang for my buck using an external USB-C SDD?