Right now we are doing a loop through Southeast Asia much like the one you have planned.
Nearly two years ago we traveled trough China with backpacks using public transport and, from that experience, decided to avoid China with our bikes. Now, having met several people who cycled through China and loved it, we are revising our opinions.
Cycling through a country is vastly different than going by bus or train. Often it is much easier as the "in between" town smooth out the hard edges of the big cities.
You might consider a loop going north from Bangkok to Laos, crossing into Southern China, then back down through Southeast Asia by way of Vietnam and Cambodia before heading back to Bangkok. From there it's a straight shot to Malaysia then on to Indonesia.
Right now we are in Vietnam where the cycling is spectacular. There are very few private automobiles so the roads are owned by two wheel traffic with a handful of horn-blowing trucks to disrupt the otherwise peaceful countryside.
, in particular, loved China.