First time, how long and how to travel?

Hey I’m 35m planning to backpack alone around end of January and I want to see alot of the country take in the beauty has to offer and not rushed,
Thinking couple days in Bangkok,

Longer in Chang mai, Chang rai, pai,
Many Southern islands like ko tao,ranong, krabi/phi phi, ko pha ngan, railay

How long recommend I stay in Thailand for and best I travel around by bus, trains, flights?

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