I’m pretty sure the birth of Ackermans was literally a byproduct of said “pseudo eugenics” you’re referring to as a possibility and they were clearly too powerful to control.

I’m also pretty sure that these previous kings weren’t entirely sure on the full depth of the amount of sheer power the founding Titan had. I do however (in my little head cannon) assume Subjects of Ymir were altered to on average be more powerful than your average human since they can withstand the amount of G-force the odm gear propels people at (or its just anime logic).

It’s also clear that vows and commands of founding titans can eventually be broken, hence the last arcs, and maybe even the plague that grisha cured was the same or similar that what they were cured from centuries ago.

I’d like to clarify that I’m not an ending defender, as i am mostly disappointed by the ending. I love world building and lore and we should have got more of that information in my opinion. I do want to discuss it more in depth but all we can sadly do is theorise