I don't mean to discourage all the work going into this, but if this is going to mainly serve as a test for the new hybrid gamemode, why not just implement it in an already-existing payload map?
Also, one possible idea is to make it so that "slopes" which usually require players to push it or else it rolls back, should temporarily disable the auto-push mechanic until it's all the way at the top, requiring a player to stand by it to push it up the hill before they can leave the cart alone again. It would help stagger the offensive team from fully committing to jumping forward during critical moments.