Is this new plan just for you? I see lots of use of the word "me" in this post. What happened to the friends? It's certainly not a good plan for first timers to the region. Maybe you need to completely shift your paradigm and plan a trip for yourself with as many stops as you feel you can stand, but then plan a separate and more rational trip for your friends, with a possible partial overlap for meet up with them somewhere before moving on separately again.

Were it my trip and my friends, I wouldn't go to the other side of the planet to spend any time at a beach and particularly not a lackluster island like Samui.

Perhaps a better plan would be to forego Samui also, then plan something like Hanoi - Halong (overnight, don't bother with a daytrip) - Hoi An (both beach + culture/history available there) - HCMC - Siem Reap. 8/9 days Vietnam, 3/4 days Cambodia. You can get to China from Phnom Penh, if they have extra time they can move on to Bangkok.