The trip south of Bangkok is fairly congested for three hours then it's awesome, the roads are really good quality and opposing traffic is separated for most of the way. Watch out for trucks and courier utes, they are inclined to move lanes quickly. Awesome scenery most of the trip. Definitely break it up, Chumpon is nice but there are numerous coastal towns worth stopovers.

Just forget about agents, biggest bunch of scammers in the country.

Different international licences were subject to accords from different years, you need to check if your country and Thailand signed to the same accord. I struck the same thing in Brazil and although it wasn't valid they did accept it out of courtesy. You also need to have accompanying required documents