Visiting first time with my wife for 3 weeks in Mai, what do you think about my itinerary?

So as the title describes, I will be visiting Thailand with my wife for our late Honeymoon, from 4th to 25th of Mai.
Our plan so far is this:

• First 3 nights in Bangkok, discover the city, visit the touristy areas, see the nightlife.

• 5 nights in Koh Lanta, in a somewhat isolated resort called "Lalaanta Hideaway resort" near the national park. We like being in a calm place but I'm a bit worried that it's gonna be hard to get food and such in that area, what do you guys think ?

• 6 nights Undecided, I'm thinking maybe Phuket in a nice Hotel, I'm just afraid it might be too crowded. We mainly want a place with a lot to see during the day, Good food, and nice somewhat affordable hotels.

• 5 last nights in Chiang Mai, see the nature, historical sites, animals…

We don't want to waste too much time traveling from one place to another that's why I have a limited number of locations to go to. But if you guys have any recommendations, insights, ideas about where to spend the 6 unplanned nights or anything at all please don't hesitate to share your thoughts. Thank you in advance.

Also to note, this is our first trip to Asia, and also our first trip that lasts more than a week, so we are pretty excited but also a bit stressed.