I honestly think this idea:
May have some merit. It's fairly simple, and it forces fleets to split up over the course of a war, and allows for a defender to have a chance to come back from a loss later on in the war, as the enemy fleets become more and more spread out.
As a thought, what if occupying (or blockading) a star system gave you warscore? You'd only get it once you'd destroyed everything in orbit in the system, and the system would need to contain a colony, but this would encourage the player to split up their fleets somewhat, since you'd need to leave behind fleets, or portions thereof, to defend conquered systems. Then the player would need to think about how many ships to leave behind, and it could allow for an opponent who is only a bit weaker in fleet power to be able to fight back, rather than loosing their entire fleet in a doomstack fight. Or, even better, you only get the warscore from occupations, if you have a fleet blockading the star system, so once I've landed an army and conquered a planet, I need to keep at least one corvette in-system to be able to gain warscore from that system. If I choose to leave only a corvette in each conquered system, the defender can then come a long with a fleet of a couple destroyers or something and destroy all of those corvettes, thus negating the warscore. If you tie this with slower fleet movement in hostile territory, that means that an attacker will need to properly fortify and defend all the systems they take, which could be done using military stations, as long as they still have that one corvette in-system. I think this may add some interesting strategic play to the war system, and help reduce the dominance of doomstacks. Ofc, warscore would have to be rebalanced somewhat with this, having a ticking warscore system would probably end up being the best, with target planets giving far more warscore than non-target planets. This would allow players to hold out with fewer planets under their control, as long as they are able to defend the systems they take.