First, lets go through the summons;

Carbuncle, either useless or completely overpowered. If it reflects nothing, useless, if it reflects all magic damage, then you are suddenly taking zero damage and doing damage to the boss. However, would it also reflect beneficial spells like heals? probably not, but if you can completely nullify a mechanic, that is broken.

Titan, AoE damage, ok, but just buffing the summoner? Not really helpful. Might see use if it buffed the whole party, but just the summoner? nope.

Shiva, freezing, assuming it would only work on trash packs, I still don't see the benefit. With proper tanking/healing, big pulls are really easy anyway and for something like this, your potency will be taking a hit, making it not worth it.

Ifrit, single target damage, simple, about the only one without some sort of dumb gimmick.

Garuda, AoE, with a knockback. Guess you hate being efficient when using AoEs. Knocking things out of range is the worst thing you can do. Just look back at HW machinist to see how annoying it is.

Ramuh, pseudo AoE. It's going to do less damage than a focused single target attack and it doesn't hit enough enemies to be useful in trash packs. Also, 30% chance to paralyse 3 enemies, which might or might not prevent damage compared to Shiva's 30% chance to freeze everything in the pack, completely preventing any damage that the mob can do. Not balanced at all.

Seraph, this is Summoner, not Scholar. If you aren't doing damage, you are useless, which is exactly what this summon is on summoner.

Alexander, again, AoE, but for some reason, this is only a 20% chance to prevent all damage when Shiva is 30%.

Fenrir, doesn't do damage, no good. I should clarify that, whilst Carbuncle can potentially do no damage, it does have the potential when timed properly, hence why it isn't completely useless.

Bahamut, basically, what we have now. Also, Summoner would never get Ultima. For story reasons, I can see no jobs getting ultima, however, if one DID get Ultima, it would be Black Mage.

Phoenix, attacks, heals AND raises EVERYONE in the area? WITH an Auto-Raise attached? Either this would be completely broken, or the damage is going to be so low, it wouldn't be used in normal play. Even if you use it to 'save' a wipe, it completely negates any sort of proper resource cost to mass revive everyone. Bear in mind, the only thing that can mass res is Healer LB3, HEALER LB3.

Odin, This seems to be the one that would be used the most. It is just pure damage with other benefits to the summoner.

Going to your Aetherflow, you want a stack of 10, which is gained over time, in some way. Assuming you want a summon out every 10 seconds, you will gain at least 1 flow per 10 seconds, tha tisn't the bit I had the issue with. The bit I had the issue with was, yo ucan gain Aetherflow by using Aetherflow. That is the same as saying, Monk gets a Chakra when using Forbidden Chakra, or Black Mage gaining a Polyglot stack when using Foul/Xenoglossy. It isn't really a resource spender if you don't give up a resource.

As for 'Big Summons' costing more, ok, but would it be worth it? You would have to weigh up the damage potential of the summons per unit of resource used and the one that has the highest damage per unit of resource will be the one that is used. Sure, you can have a cooldown on each summon, but at the end of the day, you have 12 summons. How many of them do you envision to be used for your single target rotation?

Also, Insistent Trance, only going to be used on the highest damaging summon, for obvious reasons.

Inkindeled Spirit, If you want to completely block the screen for everyone, sure but again, strongest summons only, or, if they happen to be on cooldown, you just go down the list, assuming they even have a cooldown.

Possession Trance, this is just allowing you to a period where you don't have to worry about summons. There will be a best one, that will be the only one used (one for single target and one for AoE). This text also implies summons won't have any sort of cooldown naturally, so you could literally just continue to summon the summon over and over. This then has you weigh the cost of the summon compared to the trance and whether the trance is worth it considering you will also have to summon other, weaker summons for that 1 minute period.

Aether-rich Summon. Noone likes RNG, especially if it is between a strong single target attack or a weak AoE. The damage variance would be too high and people will complain about it.

Overall, there has been zero thought about this at all except, I want ALL the summons. OK, you can have the summons, but how are you going to make them useful? Why should I care about Seraph when I can summon Odin or Bahamut. here is no plan here, it is just stuff put together with no concern as to how it would function in combat at all.