I'd rather Carbuncle to no longer be a pet for the job at this point. Making it into a stance-like buff that is required for using summon-based actions would personally be more palatable than having a pet that's barely does anything while constantly desummoning/resummoning on the field. Lore reasoning? Maybe Carbuncle persists in your grimoire/index because it's a "special arcane construct" that is unique in doing that and attuning to your weapon is now just that easy for arcanists. Who knows, lore doesn't feel like it has much importance for the job anymore anyway.