If you’ll allow me to backtrack a bit, I forgot to mention that I did eventually find a decent version of khao soi in Chiang Rai. Sold at the side of the street under two gigantic trees, Somkhuan Khao Soi does a pretty good bowl, his broth having the smokey flavour of chako, a dried spice that some vendors use in their curry paste mixture. Khanom jeen nam ngiaw is also available.
Khao Soi Somkhuan
Th Singha Khlai