Diplomacy needs to tweaked / fixed, I think. It's unbalanced because it's not give and take... it's just give, give, give. The NPCs don't have to do jack squat for you, but you have to run missions for them and give them gifts to get on their good side?
The first part I agree only on the tweaking bit, but have you played the same game I have is the real question. With enough rapport the AI will aid you in fights and even follow you around systems, I am also pretty sure it might go to where you ping at. So there is balance, that and they give you friggin' resources for doing those quests. There are also researches to min-max those quests allowing you to skip the ones that do not favor your situation.
Perhaps there's a strategy for completing the missions? If they ask me to fight something,
often that opens me up to attack by other AIs while I'm doing it. Even if not, when I'm asked to kill a number of ships, I can chase them all over the map (losing forces piecemeal on the way) and often fail to pin them down fast enough. A couple of failed missions and my would-be friend is declaring war on me and there seems no way to turn that around.
Don't chase them. Go hunt down LEVs or if you are chasing anticipate and hyper jump before they do thus when they land they are immediately being killed on sight. Use the researches to keep a friend, they'll stay with you all the way at the 30% marker I think. Failing that the 45% marker.
The missions don't require you to send your entire force, if you care to do it then send perhaps a cap ship alone with a small escort to hit and run the required things. Keep the majority of your fleet in your own base. The AI isn't going to be like the human with 3497398473 turrets pointing in all directions or mass hangar bays. Obviously if they're asking to kill civilian structures go after the extractor, ships then go after the LEVs/siege ships due to slowness and fragileness, and if they ask you go after tacticals then you just have to suck it in and kill .
That being said, the research option does help. Every race has one I think.