The story doesn't really go into detail into how Zodiark 'works' outside of a rough explanation that as a darkness primal (governing activity and growth), He speeds up the celestial aetheric currents surrounding the planet. Perhaps he does it by increasing Etheirys' angular velocity to enhance the Coriolis effect, similar to his Astral Flow boss fight arena mechanic. I can only guess at how darkness affects growth. Does it boost cellular replication, resulting in shorter generation times (and shorter lifespans, with an increased carcinogenic effect?) Either way, we can only guess at how he works. I'm sure that it is more nuanced than 'granter of wishes without any limiting condition' (unless DRK is getting a Wish spell next expansion).

It's also hard to say how well the Ancients actually understood how he worked. At the very least, this was the first time that creation magic was executed on such a massive scale. So while they had a theoretical framework on what they were trying to achieve (speed up celestial currents using the darkness aether), they wouldn't have had any prior experience with it. And to be fair, the Ancients did a lot of things by blind experimentation without understanding the consequences. Remember, the Ascians created the Void completely unintentionally, denying themselves a rejoining. They're definitely not all-knowing.

Either way, the mechanics around Zodiark's summoning haven't really been explored in the story in any depth, so most discussion around Zodiark and the solutions that he provided is much headcanon territory.

If you define an ideologue as merely someone who subscribes to a set of ideologies (it's not quite the same, as the word usually has a negative connotation attached with regards to dogma and zealotry), then everyone is an ideologue. I do think that the Ancients as a group were very much focused on 'concepts' and principles over individuals. You can see this even in their governance, with Convocation members ruling over abstract ideas like 'rhetoric'. There's a really nice segment in Shadowbringers (Lv. 77: 'Return to Eulmore') that highlights the differences between our current society's thinking and the Ancients quite nicely. To Emet, people who aren't Amaurotian aren't 'truly alive', and so it isn't murder to kill them. You see a similar line of thinking when you first enter Elpis as well, where Hyth makes you a cloak from petaloudai and you have the option to question him further on that (i.e. 'You turn living beings into clothing? Just like that?') and his response is along the lines of 'We can always make more petaloudai.' It all makes sense, on a purely logical level. But a mind all logic is like a knife all blade. Aetheric density isn't the only reason why they would have struggled to use dynamis.

The Ancients are very much written as a society of gods, a point rather bluntly reflected in their naming choices. On a very superficial level it sounds exciting (after all, who wouldn't want to have so much power at their disposal?) And then when you start to talk to them about their views on life and other people, you quickly realize that society as a whole is much better off without it. Or not, if you're not the sort of person who gets weighed down by silly things like emotion and sentimentality.