Basically there are four forms of summoning so far
There is primal summoning which is what the ascians taught to the beast tribes to create havok, this is the destructive form of summoning the warrior of light fights against

There is egi summoning which we do by pulling the aether that rubbed off on us during primal fights out of ourselves in an elementally attuned area and giving it shape using formulas, this being limited by how much aether one person can hold, the reason we cannot summon a new egi is because we are already at our limit due to absorbing ifrit, titan, and garuda's aether thus there wasn't room for more. These egi's draw off of our aether and what they already have contained in themselves.

There is the summoning that the ascians use to bring out voidsent to fight for them.

And there is channeling a primal through yourself using the power of the echo, Lady Iceheart uses this to become Shiva, we use a more limited version of it to go in to dreadwyrm trance which is why we summon parts of bahamut that form over us temporarily, basically the dreadwyrm trance is the closest we have come to
"true summoning" as we are effectively being temporarily possessed by bahamut to do his moves just like Iceheart becomes temporarily possessed by Shiva.

It's possible that there are other ways of summoning primals that would be less harmful but we don't know about them yet since all we have seen are the ascians methods.