Planning first big trip

Planning my first ever big trip and I'm picking Thailand. I'm a 32 year old single man from NY. Looking to leave 12/23 and come home around 1/8, 1/9, 1/10. So will be there for NYE.

Looks like I can pull up 17 days and stretch it to 19 if I really needed to.

I was thinking of doing 3 days in Bangkok, then deciding how to split up time between Chiang Mai and Chiang Rai. I've read I could do Chiang Rai as a day trip from Chiang Mai. Is it worth staying over night in Chiang Rai?

The harder part to plan has been the southern portion.

Will probably for 6 or 7 days up north and it seems like that would be enough so I'd have around 8-10 days for enjoying island life.

I really want a good mix of really scenic beautiful surroundings, activities and adventuring around, beautiful beaches to chill on and an awesome social scene since I do enjoy going out and having a good time.

Originally wanted to do FMP but seems like you have to stay there at least 5 nights and sounds like it's really not that great and there are other amazing parties. Then thought maybe just stick around Phuket and Koh Phi Phi but just not sure.

Will take any advice but really looking for some guidance on the southern portion. Which islands are good for what and how long should I stay at each?