I think a simple Supply System would suffice to model the difficulties with logistics and campaigning far into your enemy's realm.
Give increasing penalties to combat efficiency the farther you go into enemy territory, ticking to worse and worse equilibrium, and give modifiers to this penalty depending on size of fleet, nearby conquered enemy planets and of course techs.
This way, until late game, you are discouraging the all-or-nothing campaigns and making it more feasible to war for the close border systems or make quicker hit and run attacks on core systems.
Give increasing penalties to combat efficiency the farther you go into enemy territory, ticking to worse and worse equilibrium, and give modifiers to this penalty depending on size of fleet, nearby conquered enemy planets and of course techs.
This way, until late game, you are discouraging the all-or-nothing campaigns and making it more feasible to war for the close border systems or make quicker hit and run attacks on core systems.
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