I'd concentrate primarily on GSM (I am anyway). I guess it depends on what you want to be able to do. GSM is going to be the fastest/ cheapest/ easiest to level and get fieldcraft demimateria/ mastercraft demimateria. That's most of what I care about right now. My crafting classes aren't currently up to the challenge of i95 primal weapons, though I suppose there'd be a decent market just for selling those demimaterias. Still, I don't plan on spamming those fights enough to even have a surplus of items to desynth.

You can take advantage of a lot of the desynth content with a single class. CUL is different and the primal stuff is spread around a bit. I would just think about what you want to get out of the system and go form there.