This should be altered so that the entire fleet can spread out across the gravity well and jump simultaneously
That is what I think as well. It does this sometimes for me but I have not figured out the "whys and hows" that causes it to work that way. But when it does happen, they spread themselves out along the edge of the well pointed toward the planet they are about to phase to. It is awesome and it makes for really great timing but this is the exception rather than the rule.
or remove the jump as a fleet button altogether.
Whoa there!

Before I knew how Fleets worked I had each group hotkeyed, phew with tons of ships that got old real fast not to mention somewhat confusing. Now I would be lost without the Fleet feature/button.