Welcome! Sorry for the slow response here. And you are in just the right place.
The concept of planets being on alert is... kinda-sorta here? The AI does make decisions based on the proximity to hostiles, but it's a little more complicated now since there are other hostiles than just you, and those hostiles may or may not own planets (macrophage often lives on AI worlds, doing nasty things without taking the planet).
The AI has a certain set reinforcement budget that goes up as the AIP goes up, and it allocates that primarily along the "front lines" of where it thinks things are most dangerous, but that's not just limited to what we would have considered "planets on alert" in the first game. I can't remember if the reinforcement budget in the first game went up with more planets being on alert or not. I'm not sure if that would be wise in this game, or not.
I'll be curious to know what you think about how it's tuned at the moment, if we should show alert status or not, and that sort of thing.
Cheers!
Chris