My take would be to have a lv.92 that indeed changes Summon X II into Summon Y, where Y is Leviathan, Ramuh and Shiva after Phenix. This would be the egi version with a Carbuncle version unlocked as egi-glamour only. Then, later (but maybe not lv.100), we would learn the Summon Y II spells with the true summon, as we do lv.90.

However, we need a different playstyle, otherwise this is just a glamour trait. For instance, Ramuh could have Shock Spikes as its Astral Flow which is an AoE buff around the caster: each time an enemy hits someone with Shock Spikes, they take damages. We would then need to find thematic Astral Flow for Leviathan and Shiva (maybe its conal AoE with large stalagmites, I forgot the name). Finally, I would like them to have a different number of Gemshine / Precious Brillance cast than the 2/4/4 we already have, something like 3/3/5 for instance.