Finn's storyline should have taken place ...
... on the Supremacy!
If he was on Snoke's ship, his storyline wouldn't have felt as unconnected.
Maybe the person they needed to free could have been a prisoner in the ship somewhere, and their misadventures could have been things relating to trying to get to the prison cell, and then to the engine room.
If they wanted to include Kanto Bight, it could have been a twenty minute segway, where they board a First Order ship going to the Supremacy.