Have you ever done a single advanced secret garden? You don't get to 'manipulate unit speeds' to influence which squad takes which path on each random node. Any one of them can take a turn off of the spiral path at any one of three random nodes and get to the boss first. The only possible way a squad can be assured of hitting the boss because of its speed is the supremely unlikely event that all of your squads take the same path on every random node they encounter.

By all means, play the game how you want. But don't try to pretend that one additional round of combat which takes place automatically takes more time and effort than changing your team composition at the beginning and the end of running a secret garden map. And if FKG was a difficult game, I wouldn't have to deride strategies that don't make any sense and only make it more difficult.