Desynthesis isn't required to craft certain items, but it helps in acquiring materials to craft these items. All of the crafting classes can obtain field/battle demimateria(except cul). Only a select few can obtain mastercraft demimateria as far as we know. Personally I would first and foremost go with gsm, you can pretty much obtain any demi from this class except titan demi and a clear winner in obtaining mastercraft demi. It is really up to you for the next two classes. Are you going to actually go back and do hm primals to get the weapons to desynth? Then I would go with BSM/ALC for the wide range of weapons you can break down. I really don't see any use for ARM/WVR/LTW except for breaking down dungeon gear for battlecraft 3s, but you can already do that with gsm. I think for the most versatility I would go with GSM/BSM/ALC. Just my opinion...