You're right we don't know how long it would take them to gather enough energy to send something on at least a one way trip. This of course is assuming that Hermes didn't go rogue and the knowledge of sacrificing half of the planet's population didn't actually deal with the root of the problem that caused the final days didn't create mass hysteria at the same time. What we do know is after something was cleared for release into the wild for the betterment of the star is they were hands off. That and returning to the star was according to Hythlodaeus always beautiful and something to be celebrated. And yet for whatever reason the remaining ancients post Zodiark's initial summoning felt they could not or would not be willing to wait for the ecosystem to rejuvenate itself.
So, why do you think those who couldn't wait for that to happen on its own would be willing to wait for enough aether or whatever they ended up using to send a group of people, or familiars, or creations to go fight something they don't know how to fight? Would they even choose to send our pre-sundered self? As at that point in time Azem had left the convocation and been labeled as a traitor? Or Hermes in a "Hey Jackrabbit you got us into this mess now go deal with your pet project"? Or do you not even think about taking said risk because they one or both could choose to sabotage the mission even if you saw it as a suicide mission. Not that I think Azem would be that spiteful.
The other thing is would they i.e convocation and any general public choose to want to use Zodiark to speed up the process and thusly condemning even more souls to live inside it? You know not allowing said souls to being able to return to the Star which was pre Zodiark something they cared immensely about. To the point people who even though they respected Venat after she retired from being Azem would occasionally ask her when would she you know go and kick the bucket? As it was going against custom to not do so after stepping down from such a prestigious position. Sure they might not have celebrated it like a New Orleans wake, but Hythlodaeus and Emet did make it seem as though it was to be seen as a joyous occasion. That is a lot of speculation with very little information to say which way it all would go as we only really know how Emet , Hermes, Venat, Elidibus and Lahabrea would probably react. As we barely interacted with Igeyorhm, Nabriales or Mitron. The rest of the convocation is very much a mystery as to their personalities. Even Logrith is a mystery as Gia acts differently than how she did before the sundering.