Super stoked to hear about your trip and I'm sure it's going to be an amazing time! Wow that sounds like one of our types of itineraries hahaha. It's always so tough to decide when your traveling so far, we live in California and when we go to Asia it's a non-stop do do do type of trip. In the end it will all be an amazing experience whatever you choose.
We recently got back from about a month trip to Thailand and we had the same expectations to visit multiple countries and areas of Thailand, but after bouncing around Thailand we found that it would be best to spend the whole time there and enjoy more than running around (we did however see many different areas of Thailand). Even this type of trip would tire out the casual traveller. I would suggest that it's best to pick a few good spots and really enjoy them. ( we don't really do relaxing beach days, but if you want those this would count even more)
Hunting for a hotel room upon arrival day in each area consumes some time and leaving an area always involves packing up everything, doing the double triple room check and then leaving your bags in the lobby for a later pickup (if you plan on exploring the day your leaving), so this takes up some time to.
If you were going to do the no Bali plan I would trade Phuket for something like Railay beach and then you could split some days between there and other places like Koh Phi Phi or Koh lanta.
If you were going to leave in Bali then I would trade out Phuket and spend more time in Bali. This is where the decision gets tough for me however, because I LOVE BALI but it would be hard to imagine if we went to Thailand and didn't see the amazing islands and beaches ( I loved Chiang Mai but preferred the islands, just a personal preference :) ).
Sorry for the rambling but when it comes to travel it's so hard because we never want it to end and the places are always so amazing that we want to see it all. When doing our research you always read take your time and enjoy there will always be times when you can go to the other place you are thinking of visiting.
We are actually heading to Bali next month and am glad we didn't try and squeeze it into our Thailand trip.
Both places are for sure on our list of the most epic vacations you could have, so with any decision you can't go wrong.
I vlogged our whole trip from day to day if you want to see what bouncing around Thailand was like https://www.youtube.com/playlist?list=PLlg2RG3ecmcvJ6Kgb8PS9UipczqJcyMuS and also will do Bali next month if you wanna see :)