Your the Admiral/Moff of this game. So consider this as a bonus.
Any repeatable tactic by an enemy is something to be learned of and exploited. Not the way you think, but rather by the way the enemy behaves.
Ok so I love it when the AI does this because it can become an area of the game where I have drained an AI of money. No really, the enemy fleets are dwindled and defending armies are small.... and I get bored and start a new game.
First is to really decide if you need to control the area of space in a system. If you are not ready to defend it, don't. If the AI has decided it wants that SPACE you then decide if you want to waist resources to hold it. In most cases its your ego that wants to control the SPACE but it is not actually useful. No you don't need to build ships everywhere not even if its a level 2.
If you must have the SPACE - only to learn about strategy - go ahead and build up fighters just to learn. Fusion and Tie Defenders and top craft will end up eating anything that comes, Viscounts SSD's dont care, they will. You will need to build 5 at a time on several stations. Then toss them all into the theater that you find particularly hard to hold.
Bilbringi is a blast! One time I gave up counting the number of Viscounts i destroyed having slowly building up Tie Defenders to defend it. In one battle two Viscounts jumped in the first wave, then another followed suite. 7+ Viscounts if I recall were churned through. I was mad as hell wasting ships on the system but ended up learning from it.
For Planets once again do you really need the Planet OR will you use it to burn the enemy armies into carbonized metal? Have your strongest and fastest (flying is good) defenders ready to rush to the landing point of the enemy. Get there and churn through the enemy.
So it is a great opportunity to learn of what units are best for defending a planet/city, or rush to a landing point to finish off the attackers. Each is different and great to learn.
Remember, you can learn how the Enemy/AI is acting so you can "distract" it by having battle for what it wants, versus you then going out and attacking what it is barely concerned with.
I've sometimes had a nominal fleet nearby that jumps into a system that the AI camps on just dumping units trying to take the planet so to have a fun time with it. the fleet jumps back out after the battle to do it again.
It can get boring and repetitive so perhaps taking over that system another time might be better.
HINT Starting a new game can bring about different results as well.
Oh yes, dont auto resolve but rather hit fast forward