I see nothing wrong with your plan as long as you are comfortable driving on the English side of the road and are a what I term an aggressive/defensive driver. Personally, I have driven a circle from Chiang Mai thru the hills and golden triangle. Have also driven from Phuket to Chumphon and back.
Main road signage is a combination of Thai and English. Small roads have only Thai signage. The most difficult part of your drive will be in the vicinity of Don Muang airport. If available, I strongly advise that you get on the expressway until you are away from the congestion of Bangkok. A good map is a must since most road signs name a town and not always a route number. A trick to keeping on course is to make note of the kilometer markers on the side of the road. They normally have the kilometers to the next major town and the name of the town in Thai only. If they disappear for a period of time this is an indication that you got off track (but not always).
People will be surprised for the most part by a Westerner driving in Thailand, but it is no more difficult than in many other parts of the world. I include Manhattan as part of that world. Away from the towns and cities there is comparatively little traffic. Be aware that mopeds, scooters, and motor-bikes are a part of the scene everywhere. Be always alert. Don't be surprised to see 3 or 4 people on a single moped. Also don't be surprised to see a song-tao (pickup truck used as a bus) with people hanging out the sides and off the back. Also don't be surprised to be following the local egg man's Toyota pickup with the entire bed loaded higher than the cab roof with eggs. They tend to drive quite slowly. Give way for bigger things. They appreciate it and rarely give you the same courtesy. For the most part, I found Thai drivers to courteous, the amount of courtesy is inversely proportional to the amount of congestion. Don't be surprised by passing behaviour and two lane roads being used as two lanes in each direction. 4w.d. pickups rule in the countryside. They tend to be among the more aggressive-stay alert.
The most difficult thing about Chiang Mai was finding the road north from the airport and finding the airport upon return.
Avis and Budget allow one-way rentals and they are not difficult to arrange. They can be done on-line. I rented from Avis in January and they were fine. I have Budget rentals this month both in Chiang Mai (just for local travel) and a one-way from Phuket-Bangkok.
The main roads are quite good and compare favorably with those found in the Northwest U.S. The minor roads range from excellent to abysmal. Minor road signage is often non-existent.
Maps:
Periplus travel maps have Bangkok, and Chiang Mai. The Chiang Mai includes the Goden Triangle, Chiang Mai and Chiang Rai. A note on Chiang Rai, I suggest the Dusit Island Resort. BTW: some lesser roads are shown on these.
Nelles "Bangok" map is pretty good.
Bartholomew "Thailand" is good for main roads.
There supposedly is a Bangkok expressway map that is helpful. Amazon has mine on back-order so I hope to find one in Bangkok.
Your plan to fly "C" on the return may not be worth it since the flight is 1hr and 10 minutes. Coach on Thai is fine and exit row aisle seating can be requested (and normally obtained) at check-in. You can also e-ticket your return ticked on thaiairways.com from the U.S. charged to your credit card. Just request an e-ticket rather than in-country pickup. One way CNG-BKK should be about $52 U.S.