As many players already noticed there exist a few side effect from for example Expeditions. When your fleet falls into a wormhole then the vespene cost to get it back to your home planet often are enormous and make it unprofitable to return the fleet.
So people choose to kill the fleet by attacking random players near the wormhole exits.

I would like to see an option / button where I can disband a fleet. This will remove the player's control of the fleet but it may stay where it is and starts operating independently or just sit there until someone comes for it.

A different scenario may be that people without sufficient wormhole technology may loose contact to the fleet due to the large distance. After a bit of time waiting for the fleet to find it's way home (waiting time based on distance) there should be a good chance of having to give up the fleet. But there also should be a chance that it may return even much later if the fleet is able to sustain survivability*. In this scenario when the player has evolved the necessary wormhole technology level then the fleet remains completely under the control of the player like it is currently.

*Survivability: When sending like 10 overlords in a wormhole they have nearly no chance to come back at all because they can't do something to keep the fleet alive. But when you add things like providers and spy probes the chances of survival should increase drastically since the fleet has a) the possibility to scan the surroundings for enemies or resources (vespene for the flight mainly) and b) the possibility to farm those resources with providers.

In other words, disband the fleet could give the fleet to pirates. I don't know why game makers didn't included the option from the beginning since a lost fleet consumes a travel slot, and sometimes sending Vespene is so expensive that makes it unthinkable.