wait a sec, doesn't this just make it polarbeam with a cart? Didn't you say that people stayed in spawn for most of polarbeam until the last few seconds?
Polarbeam essentially only has one set of timers. This in a roundabout way has two sets (the top KotH timers and the cart [only kind of a timer in the way after a set time it makes the round end]).
Polarbeam's issue was the fact that there really wasn't a way to stop stealing the other team's work. This still needs a bit to counter that, but a reliable timer alongside a cart that does move back should make it not frustrating for either team.