It's not clear to me if you have ever been to Thailand (or Asia) before - that would be helpful in calibrating responses. Based on your post, I'm going to guess "no". I will offer a few tips here, take them or leave them as you see fit. FWIW, I've been to Thailand many times, was just there about 2 weeks ago, my wife is Thai, we have lots of family there...and I was just there so have some fresh news...
First of all, stop deluding yourself. You do not have 8 days in Thailand, you have 6 at best, and the first day after arrival you're probably going to be pretty wiped out by jetlag and the fatigue of flying half way around the world. On top of that (with your 6 usable days), you're talking about flying domestically twice (Bangkok to Chiangmai and back; Bangkok to Phuket and back). Even if you're skilled and experienced, those (four) days when you're flying are going to be mostly consumed by travel, too - so now you're down to, what 2 or 3 days when you're not arriving or departing on a plane?
I know this isn't what you want to hear, but your plan seems totally unrealistic to me.
I just got back from a very, very short trip to Thailand. I was there for about 9 days, not including my arrival or departure days. I think it's generally pretty crazy to go from North America to Thailand for that short a time, but I had some unique circumstances that allowed me to justify it this time. I also flew in First Class (lucky me, it was awesome), so I was able to get really wonderful, solid, restful sleep (perfect conditions), all along the way, so upon arrival I felt 100% great. If was trying to tough it out in coach, I would expect to completely write off the next day as needed time to recover form the miserable flight. I'm also super-lucky that all I need to do is get there and step outside the airport - then I'm met by family (locals) who whisk me away and take care of every possible thing I might need or want, as long as I'm there and until they drop me off at the airport to leave - and I mean that literally. I do not need to worry about anything, just east, drink, sleep, and everything is done for me. Zero language barrier. No challenges navigating, I never have to even think about how to get from point A to point B. No worries about cultural leaps (they know I'm a clueless foreigner, so they tell me what to do, how to act at temples, when there's a monk sitting in front of me, etc.). For me, it's easier than being at home.
If you have not been to Thailand before, or another "very foreign" country where the language, written script, culture, local customs, etc, are completely alien, you may find that very simple things (finding your hotel, a place to eat, getting somewhere) will take longer and require more thought/effort than you expect. It's also stunningly hot and humid there - don't discount how much this can impact your efficiency. If you're familiar with Thailand, then ignore some of the above.
Now, I would not want to discourage anyone from visiting there - tourism is a major industry, things are very well set up for foreign visitors, Thais are wonderfully gracious and welcoming to guests, etc - I'd say Thailand is one of (if not the) easiest Asian country for first-time visitors. But if it's your first time, don't expect everything to go quickly, easily and efficiently.
All that said, I still think your schedule is completely unrealistic.
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