About 10 years back, maybe less, drove Bkk - Nakhon Sawang - Mae Sot - Mae Sariang - MHS - Pai - CM - Sukhothai, spending a night or nights in each place.
The 1st part of the Mae Sot to Mae Sariang road is very scenic in a river valley. It then turned away from the river and became quiet remote, but was a really enjoyable drive. We always regret arriving Mae Sariang late afternoon and leaving the next morning, as we should have spent 2 nights there.