Yeah, the one giant armada thing really is the best tactic. I keep one giant armada until I destroy the enemy's giant armada, then I split it up to blockade all their planets.
A combat readiness system would be interesting. It could be based off supply limits, and reduced readiness would make the fleet move slower and start to get combat penalties the lower it goes. A fleet above that limit would start to degrade readiness depending on how far above it they are. Just having ownership of a system would increase it a bit, with extra for each added base built there (defense or mining). Planets would have a significantly larger boost depending on their population, with a significant bonus for each level of spaceport. Enemy base presence would reduce the limit, and supply bases could be built in enemy territory to provide a staging point for larger fleets.
So the big fleets would only be able to maintain readiness over your worlds with spaceports, with raids deep into enemy territory either done so at reduced readiness or after you set up a few supply stations.
The idea would be that smaller fleets within the supply limits of enemy systems could fly in and bust up enemy systems a bit and make a path for your bigger fleet to come siege their planets.
Making an AI that intelligently used the system without being mercilessly abused by the player would be difficult, though. It would have to dynamically respond to threats and split forces and call for reinforcements as they detect fleets entering their systems.