So I am trying to make an ally who will take forward positions as the game progresses and maintains them. Some defenses and basic buildings and specific locations.
I tried to preplace the buildings, run AI scripts and destroy them to see if the AI will take the positions back, but I couldn't find AI which actually rebuilds lost buildings.

Do any of you know an AI that can do so or some kind of trick to make AI build specific buildings in specific locations?

I am aiming for natural feeling, with it being actually built and needing protection during this time + AI rebuilding it with time when destroyed. I can make it with triggers, but it will be time consuming and simply ugly.

You will need to resort to modding, using the AISE plugin to create townlets like that on specified locations with the `create_script` command. If you do, this can be made to happen pretty fluidly.