I have this once a lifetime opportunity to go to southeast asia but only 6 weeks to see "everything". Starting from Bangkok to Kuala Lumpur to Hanoi to HCMC to phnom penh and Siem Reap and back to Bangkok and Chiang Mai does sound unrealistic... for some? Thanks for yours words of advice.
It's entirely possible to do several countries in a fairly short time frame like that. A major consideration is your budget: how much do you have to spend, and how much of it do you want to spend on traveling?
How much do you want to see of each country? Will you be staying mostly in one city per country or seeing different parts of each country? Are there specific things that you want to see in each country? How long do you think you' drive to spend at each place?
My husband and I traveled through Europe, six countries (eight cities) in fourteen days. We spent one or two days per city, and traveled by train, sometimes over night.
Looking at the map, I think you can totally do it. Just keep it realistic and flexible, if possible, as to how much time you plan to spend in each country, and how much you want to see there. And have fun! almost 6 years ago