As soon as France and Spain got involved, the Patriots went into a defensive position, reacting to British moves rather than being aggressive. This came as a shock to the French, who thought they were joining a war close to being won. Hindsight being 20/20, this was a good move for the Americans, and a bad move for the French. a more aggressive US could have shaved a year or two off, or doomed the revolution if the aggression didn't go well.
You need the French/Spanish to get involved much earlier OR the British to acquiesce much earlier, if you want a mid 70's resolution. Patriots by themselves aren't speeding up the timetable that much.