It's sorta silly but it works. You just make a small or tiny ship ( tiny is best cost wise..) that has nothing, no weapons, no modules, no defense etc..  you can give it any extra stuff you want. Then you just take a planet and mass produce them. If the AI is leaning towards Hostile with you send them a couple and it will improve your relations with them. Plus side is they don't cost you an arm and a leg. They can't be used against you ( well they can but it's just a 1 hp ship which can't attack or do anything so it's just target practice.. )