What if, and hear me out, they made SMN like MCH and DRK? Here's how it would work.

As a arcanist you get carbuncle. It stays with you always. Sure you can have red, blue or yellow carby but it is your pokemon through and through. At lvl 30 you get SMN and Ifrit but, he's just a oGCD that works like DRK's summon. He does a set rotation of abilities and them ends with a big attack like MCH does. Kinda like how summons work in FF7R. All the while Carby is STILL out. So you have 2 summons going at the same time. Now Ifrit, Titan and Garuda can't all be out at the same time, but you as they attack they fill a gauge, one that your pet actions that tell carby to do things also fills. Once that gauge is full you can then summon Demi Bahamut first, who does his own rotation, again like DRK, and ends with a big attack, again like MCH. Making you able to have 3!! summons out at the same time. Then once Bahmut is out of the way then you summon Phoenix the next time the gauge is full.