General Question(s) on Group content and midgame

[Estellios]

The most useful desynth skill is Goldsmith because it's one of the few options for Mastercraft demimateria which is used in getting or crafting almost all top-tier crafting gear. Culinarian itself can only desynth fish which can get some rare materials but only from unbelievably rare fish, and then the materials are only for vanity items. I don't think it's worth it. Other than that Leatherworker is currently very useful for getting Fieldcraft 3 demimateria which are also used for getting high end crafting items, but it's not as necessary.

That said, having Culinarian leveled up will be incredibly useful as making HQ Spicy Tomato Relish to turn in for Artisan's Spectacles for desynthesis is the easiest way of getting Mastercraft Demimateria.

If you aren't considering going hardcore on crafting you don't have to deal with the nightmare that is demimateria so just feel free to level up whatever desynth skills you want. Getting materials from random dungeon drops and aetherial gear is useful on its own.