What is the advantage of fleet detachment (aside from doing multiple actions at a time like scouting or commodity trading)? Let your ships run down enemy fleets they could not otherwise catch? Okay, nice! More loot for me!
If the enemy can do the same to me, then it probably is never safe to use big fleets until endgame when you can afford a fleet that can kill everything.
This is why I said it'd be best if you could attempt to disengage from an attack by a detachment regardless of fleet size. In what way is the rest of what you describe a bad thing, anyway? If you're focusing on non-combat but aren't using combat ships for escort, why wouldn't pirates and such take advantage of this?
Due to battle map size limitations, there is no way you can disengage after your fleet grow big enough, as long as the enemy wants to fight and can. That is just not possible. That is why the game forces normal battles when fleets are too big and the enemy wants to fight.
Currently, if the enemy is bigger but slower than you, you can run away. If they can split so that the new faster fleet can catch you, then it is just like today when a scout catches you and sucks in all of his friends within half a screen away, and all fleets effectively become one super-sized fleet. You only run away if you think you will lose. Otherwise, you would just crush them all on the spot.
The point here is, let's say you're evenly matched more or less with the detachment, but the rest of the enemy fleet outguns you by a huge amount. Going with the option of attempting to wipe the detachment or force them into retreat will be a lot messier under the time pressure of doing so before their main fleet arrives. Still possible, but you'll have to put your own ships at greater risk in combat to be successful.
Extended battles would need to be changed to support this. Currently, they all show up at once, up to battle map size. The exception is you. You may join in with allies already on the field, or allies may be deployed a few seconds behind you. If you are forced to fight a normal battle, the delay is insignificant.