Pen BKK is a great hotel.

But, if it is an option, I would suggest staying three nights at MO at the beginning, and then eliminating the one nights stay at the end. One day is not enough time to do anything anyway.

If a one night stay is necessary due to an early morning departure from BKK, I would choose Anantara Siam or Grand Hyatt Erawan for the final night for their convenient location.

Finally, I would do either Phuket or Samui, depending on the season. A 7+ night stay at one resort is much more relaxing than having to move. Then, you could redistribute other nights for a longer stay in Chiang Mai, or to include FS or Anantara Golden Triangle, for example.

If Phuket, try to take advantage of the direct flights TG operates from CNX to HKT on certain days.