Rate my intinerary for 3 weeks

Hello everyone! In January I'll be traveling to Thailand with my partner. We're looking for a lot of nature and culture and really want to avoid crowded/party scenes. Here's what we decided to do. What do you think? We will be packing light and we're ok with being on the go (I'm a bit restless) but don't want to be rushing too much. Obviously there will be some internal flights:

Bangkok (2 nights)

Chiang Mai (3 nights) 

Nan (3 nights) 

Chiang Rai (3 nights) 

Khao Son (3 nights) 

Railway beach (2 nights) 

Koh Mook (4 nights) 

Back to Bangkok