- There is a maximum limit to planetary bombardment. My fleets hit it at somewhere around 30k power? (can't remember exactly), and I discovered I could split fleets and still have the same bombardment/day. This means that yes, once you go into late-game when fleets can easily brush aside fortresses, it is MORE efficient to split your fleets and invade from multiple fronts. If your enemy keeps their fleet in a doomstack and tries to hunt down your fleet A who keeps running away, then your fleet B get free sieges + invasions + war score.
Bombardment gets quite pointless later on. A good Psionic/Clone troop army of 20 can just drop down there and win the invasion without a problem; all you need is to escort them as they go in, cap the planet and get out. It saves you lots of Terror Bombing trouble too
Depending how fast other fleets can FTL and reinforce each other, maaaybe it's good to split a bit. Especially if all your groups have good admirals. Just to bait them and start a fight on your own terms, especially if you know the FTL entry-points in the system. You risk much, though, as you'll suffer lots of losses by the time everyone's there and the fight begins in earnest.
I'd advise sticking together as a solid group, and force a decisive fight on your own terms before you can safely split up!
Personally I don't like how the combat is... forced? They get close enough to each other and then they just aggro directly charge into each other.
I'd love it if you could still retain control of the fleet's movement direction. Two fleets could strafe each other and shoot at what's in range as they move, with formations maybe adjusting a little to face each other while strafing. Faster ships could kite and disengage. Combat could end as soon as the two fleets both decide to walk away from each other. If you are close enough to the system edge, you could even have your ships even start to prepare to FTL away normally. You could keep your fleet in the Capacitor/Repair field, or out of the Quantum Destabilizer, those'd be some sort of uphill and downhill battles.
It would be nice to get transports out of trouble. Also Covettes and Cruisers would only preffer to rush into the enemy if the suggested movement route is towards the enemy. Otherwise they'd stay behind the BB's. It could make for calling in a well timed charging attack on the enemy fleet.