Note that switching district types is fairly painless, so you don't have to worry about the city districts until you actually need them.
This + the ideal is to have some kind of housing bonus (traits/slavery/civics/traditions) such that you can fill your building slots with workers while running mostly resource districts.
A generic city planet isn't good to aim for IMO, that's only +2.5% for specialists. Better to get forge world and co. for +5%. It's the resource gathering specializations that are harder to judge, since they are by design limited by the planet.