First time in Thailand - 24 days itinerary questions

Hi, redditors, my girlfriend and I are preparing for our first trip to Thailand and Cambodia. We are super excited but planning can be a bit overwhelming! We've never done a trip this big or far. We are travelling from Spain for a total of 24 days in August. Our idea of an itinerary is this:

Arrival at Bangkok on day one

Visiting Bangkok for the next three days

One day trip to Ayutthaya

Bangkok - Chiang Mai the next day

Visiting Chiang Mai for two days

Doing a one day trip to Chiang Rai

Flying from Chiang Mai to Siem Reap next day

Visiting Angkor Watt for three days

Flying from Siem Reap to Koh Samui the next day

Spending 10 days in Koh Tao/Koh Samui/ Koh Pangan

One day to travel to BKK, and then we fly to Madrid on the next day.

We've read that in August is better weather-wise to visit these places on the south, but we're not sure if we should just stay in one island all of our time in the south (the idea is to relax there after a intense travelling through the north and Cambodia) or half in one and the other half on another. We're also not so sure about which island to pick.

What do you think about the itinerary? We're open to suggestions and to make changes to it.

Thanks a lot!