going LM chiang mai and chiang rai now for five years, and i have only taken the bus the first time. It was a horrible experience
I suggest renting a car, there is a lot to see on the way as well
I've taken the bus and survived. Also to Mueang Nan... .and survived.
Your point is well taken though, having your own transportation in the north is convenient and liberating. For OP they'd be hit by a o/w surcharge on the itin described however.