About the fleet logistics
It doesnt even has to be in relation to planets or stars. Just make it unlimited/ but negative rate. Means when you go over your max fleet supply you have overproduction, means neg economy. Means you start to pay for every ship that is over limit. Means you need a good economy to support it. If your economy isnt stable or rich enough than you have to get rid of them again.
And for a big empire ( multiple star systems), i really think its needed. Because when you only have one star with 60 planets, you will always find "strategic places" to defend and hold in order to keep the rest of the empire safe. And with a planet around you have a lot of support. But a sun is bad. Even when you put there a big fleet it is easy to sweep through and go for the next planet. Also your fleet doesnt get any support. And always have to circle around. Sure you can set up fleets in each connecting planet. But in my case the sun had 5 connections. Make five defense zones. So five times more amount. I had the cash and resources but couldnt. My enemy on the other hand only had a sun with two connections. And when you calculate all this up for five stars you have to keep safe, than you you loose a lot of offensive force.
What i did was, putting small fleets in each conquered sun and two big ones in my middle star system for support. The other big ones i used for attack. while attacking i was always zooming in and out of the universe. Watching for some freaking 20* siege frigate starting to go for one of my stars, and than try to send one of my big supporters over. So in the end it was a lot of flying around. Back and forth. Sometimes they get through take one planet out. They leave again. Because my big one arrived and they are not strong enough to go deep in my system.
On the other hand, when i had four sun-systems, and the other four players were divided on two star-systems. It normally should be pretty much done for them. Unless they would make an alliances aklltogether. And if they would and do good smart attacks, I wouldnt be able to keep defending without loosing my attack power. Because i cant support my big empire anymore as i have the same amount of ships as the others, but have to support a much larger area. Means i have to retreat and than we would where we had been. At the beginning of the game.
I was also thinking. Can you actually degrade your Fleet Supply? Because if not and you had a big empire, up to full Fleet Supply, and get crashed, your finance might brake.And not be able to recover at all. Dont know just occured to me right now.
In fact all this doesnt bother me that much. I would appreciate some changes, but when not it is still OK.
But the diplomacy really needs some upgrades.
I mean money rules the world. So why i need missions to persuade someone

Everyone is happy when getting a transfer.