Hi Jen
Good Morning
My area of expertise is Thailand, primarily Chiang Mai & northern Thailand but I will try and offer guidance on the other areas you want to visit.
If you want to finish on Koh Phangan then I would suggest the following:
Bangkok - Chiang Mai - Laos - Cambodia - Vietnam - Bangkok
-snip-
When you leave Chiang Mai you can travel by bus to Chiang Rai & then cross-over into Laos overland, to travel from Laos to Cambodia I would suggest either by train or bus. From Cambodia to Vietnam a flight is probably the easiest route to take, then when you fly back to Bangkok you can take either a bus or the train down to the southern coasts & islands.
I would recommend you leave time in your schedule to visit the Andaman Sea places, as they are without doubt the most beautiful in the whole of Thailand !!. If you use Phuket as a base for this part of your journey, you can then EASILY visit Phang Ngaa Bay(James Bond Island), Phi Phi Islands (The Beach), Krabi, Koh Lanta, the Similan Islands(the best scuba diving & snorkelling in Thailand) & of course Phuket.
On the Gulf of Thailand coasts(same area of Thailand) you have Koh Samui, Koh Phangan, Koh Tao(scuba diving), then as you head back up towards Bangkok you have Hua Hin & Cha Am(coastal resorts), then beyond Bangkok on the eastern side Pattaya, Koh Samet, Koh Chang, Koh Kood.
In terms of time allocation, I would suggest:
Chiang Mai & northern Thailand - 2 weeks
Laos - 2 weeks
Cambodia - 2 weeks
Vietnam - 2 weeks
Andaman Sea - 2 weeks
Gulf of Thailand - 2-3 weeks
Leave a little extra time if you want to explore Bangkok, but that requires just a few days.
With a little extra research you should be able to do ALL of your Thailand travel WITHOUT stepping onto a plane, by a mixture of buses, trains,taxis, ferries & of course walking!.
I am always happy to offer more assistance if you have specific areas within Thailand.
Kind Regards
Kevin
Moderator comment: please see Forum Rules
[ Edit: Sorry, no promos please.Link to Forum Rules added. ]