Just to clarify but what we are told ingame is not that Dynamis is an equal force to Aether, even with those said 10 planets, Meiteion still couldnt pierce through the celestial barrier and had to work slowly leading to only piercing the weaker parts of the barrier while gnawing on the rest, it sounds like Dynamis is extremely weak against Aether but with enough Dynamis and enough time, you can slowly chew threw a barrier of aether but it still wouldnt be able to beat a concentrated amount of aether unless the piece of aether just stands there doing nothing for years.
So the fact stands that Aether>>>>>>>Dynamis.
You are correct to say Ancients wouldnt be able to compete in Ultima Thule since we learned that Ethyris was especially strong in Aether while Ultimate thule is mostly devoid of it with Dynamis being the dominant form of power, but like I said again, you only need to be creative, what if a huge source of Aether was moved to Ultimate thula, in a very similar way to how we moved there, it isnt unlikely that the ancients could have figured out something similar, and since they were able to use aether in a far more reliable and effective way they would have been able to be a lot more deadly to meiteion.
Now the main task at hand would be tracking meiteion, you would need either somekind of spell or more likely a creation, obviously not more entelecheia but more concepts that might be able to detect and manipulate dynamis since the ancients couldnt.
Again, with the power of creation at hand and nigh infinite time, there arent any problems that cant be solved, but its a video game at the end of the day so they have to make the good guys win.
Just a reminder that this is a video game and that story does not make for a meaningful argument about one's own existence by saying everyone is like X, this is a written story and you cant use it as actual lessons.
Especially since it is highly unlikely for an advanced society like that to still deal with existential issues when answers for the reason being even have such questions are internal, and once understood, the answers to the questions hermes kept asking become irrelevant, maybe a society of Hermes would end up like that, but not one full of Emets
Um nowhere is it said that only Hermes is able to create entelecheia, he was the only one INTERESTED in creating them hence he did, that doesnt mean others are incapable of doing the same, after all the power of creation is simply up to your imagination.