@Tim ******
Why?
1. More and more lockdowns. I would not be surprised if they stop all domestic flights starting on the third of August, making it very difficult to travel around.
2. In the Deep Red zones, all the tourist sites are shut down. I think most of the markets are shut down too.
3. I don't think there's any way easily to get from Phuket to Chaing Mai,, and I don't know what's still open in Chiang Mai
IF the OP is coming to Thailand, not as a general tourist, but instead of someone who wats to stay here long term, OR wants to stay in Phuket, I think it's a great idea. Otherwise, postpone it.