First of all, don't worry about them stealing your idea when they can't even get their own off the ground.

Secondly, if your working relationship with the founders is "let the lawyers decide" you don't owe them anything, much less any more time out of your life.

Thirdly, staying on longer would give them room to maneuver you into a bad legal position with respect to the non-compete clause. If you give final notice of leaving and the next day the company unexpectedly pivots to your idea, you're left in a bit of a jam. Sooner might well be better than later in this case.

What you decide to do is ultimately up to you. But be wary of being manipulated into another 4 month notice period, and another, and another. Your time is the most important resource you have (up there with your health); be wary of wasting it.