I decided to use the trance system introduced with 3.0 for SMN and extend on this also i don't use any pet at all.
SMN gets Carbuncle as an early "lesser" or "normal" summon and with levels and jobquests he can unlock up to 3 slots for "primal" or "Egi" summons.
With additional sidequests SMN can unlock more primal summons, some specific for PvP, or PvE or for both.
In short, SMN summons an egi to enter the corresponding trance for X seconds, like Bahamut Trance. The summon gives the SMN a set of summon specific actions (basic actions, trance consuming action, trance ending/signature action), on a second parallel GCD, like the summon has it's own GCD.
The actions of the SMN himself are more or less some DoTs and low damage filler abilities with the main goal to support the summon.
To provide a variety of used and useful summons you can't use the same summon twice in a row, so you have to cycle throught at least 2 different summons.
Every summon causes a utility/buff by its summoning.
If you use an action from your summon and it hits an enemy with DoTs/debuffs from its SMN it will extend the duration of them.
With the loss of the summons as pets, their is also no tank pet.
Every summon will instead be somewhere between "full AoE damage" and "allmost no AoE" with some variety in AoE size and supporting actions.
Also SMN itself will lose its range and will fall on the half of the original range by also losing most of his casttimers.
After encountering the first primals the Allagans needed something to prevent their troops to get tempered.
The invention of choice was a soul crystal with the power to absorb corrupted aether and purge the wearer from it.
After extended tests the allagan scientist found a way to use the stored corrupted aether to manipulate the summoning of carbuncles and created the first egis.
As it seems the egis, less in power than their corresponding primals, can temper their SMN by long enough time of summoning. The soul crystal gets numerous additions and adjustments to make it more save to summon egis and use them in battle.