This is not true, they need more energy as they get older which hey can get through various means. The amount of blood they need doesn't seem to actually change, they use the blood of others to stabilize their genetics.

I think you can notice the contradiction here. Energy isn’t really the problem, you can give them as much mana as you want, a 1000 year old Apostle will still eat a building worth of people. You aren’t wrong that can gain from other methods, but to be specific those methods are alternative food, aka magical creatures and other sources of blood, that is not optional and I am not sure what gave you that impression.

How are they even going to do that, as the only ways to become an ancestor are to be made one directly by a true ancestor, or consume an ancestor?

There is many ways to become an Ancestor, the easiest is getting your Idea Blood from a True Ancestor (or at least Arcuied, we dont know of others and she is a freak). Its completly possible to become an artificial Ancestor, you just need to create a powerful enough curse (Idea Blood) to "upgrade" yourself