Perhaps the fleet upkeep should go WAY up with the distance from the closest planet with a certain structure built? This way preventing the fleets to go on indefinitely and destroy all in their path. Also simulating the supply line I guess. The colonising could not even be restricted by culture (culture could have an effect on pirates or income of the plante though) since even with the colonisation the fleet could go no further than before untill a "certain supply structure" - I guess one or the other type is already suitable for this - is built.
Just an idea.
Actually, I kinda like this idea. Let's make it so, that when (for example) fleet support lvl2 (the existing planet upgrade, I'm not sure about its name right now) is build on a planet it decreases ship upkeep from this planet on.
I.e. if your fleet is one jump away from a planet with lvl2 fleet, you pay like 1 credit per click per ship, two jumps it costs 2 credits, 3 jumps 3 credits/(click*ship).
This would simulate fleet support over long distances. And lvl2 fleet support would be necessary for the infrastructure to support a fleet.
So, if you have a large fleet entering deep into enemy territory, this could bankrupt you quite fast if you don't have the credits to pay for it, subsequently slowing you down. This would force you to colonize planets on the way and building them up a bit, before you could advance further.
But if you would be very rich and have a large account, you could still attack deep into enemy territory, you would just have to pay for it, giving you a bit of a disadvantage against the defending player.
How about that idea?