I've lived in Udon Thani Thailand since 2005 and have cycled extensively through Thailand and the neighboring countries. I rarely use commercial transport as cycling is just so much nicer for me. There are tons of decent places to stay all throughout the country. Thailand is miles ahead of its neighbors in this area. I've never had a problem finding a nice clean place to stay for the night no matter where I was at the time and I'm really picky when it comes to finding a place to stay. No need to camp at all when decent accommodations with AC, fridge, tv & warm water shower run about $12US/night. Many of these small motels (called resorts here) also have a restaurant and some may even have a swimming pool. Sure there are seedy places as well but they're easy to avoid. Hotels are located in every major city/town or tourist area as well. i looked at your proposed route and can safely say you won't have any problems finding a nice place to stay for the night. Please note highway 1 is very busy. Try to avoid it if at all possible.
I've only cycled in Bangkok once and that was on a Sunday morning when traffic was relatively light.
Traveling from Bangkok to Chanthaburi will probably take you more than the 5-6 days you allotted yourself unless you plan to use public transport.
Thailand is not Washington so you should expect for hot days which may limit your time on the bike. The rainy season should be finished by then but that means clear skys and that sun can be a real bear (we're about 15 deg north of the equator)
Hope this helps.