Hot take from someone who hates current SMN: Carbuncle is fine.

IMO, Carbuncle serves its purpose quite well with the redesign of SMN. It's your companion for the entire leveling process and is the entity through which you channel more powerful Summons as you grow.

Mechanically, Radiant Aegis is an amazing tool in higher level content and the only reason it has two charges is because it's balanced by the current summoning mechanic. You axe Carbuncle and you kill that second charge, making SMN and BLM have identical personal mitigations.

If you are lamenting old pet class gameplay then I feel for you but that playstyle is gone and it isn't coming back. Perhaps Beastmaster will bring back some of that flavor in some capacity, but it's dead and buried on Summoner.

That being said, there are some minor things you could add that bring back some of that flavor, however they would really only apply to higher level content. For example, since a theme of SMN utility is that its restrictive you could have a Passage of Arms effect unique to Carbuncle where it creates a small, localized shield that effects all party members within. This gives you a party mitigation similar to Red Mage but restricted by needing Carbuncle out and having to place the shield on the map.