Firs time Solo Travel

Hello, I am planning my first solo travel this july and I was looking for SE Asia as an option. I'll be on a tight itinerary so any changes or places that are not worth enough i will be wiling to change things. My plan is to go 2 nights sin Singapore then fly to Siem Riep and stay in the area for another 3 nights, then flying to Hanoi and staying there for 5 nights to explore hanoi, ha long bay, sapa... after that flying to chiang mai for another 3 nights and finally to bangkok another 3 nights.

My concerns and questions are.

1. Since it is my first time solo traveling, is it a good place to start?
2. How safe are the places around there?
3. I saw it is usually rainy in July, how bad is it really?
4. Im more of a nature guy that's why i only choose 2 nights in big cities like singapore or bangkok, however are they worth it or should i add more days or remove from other places?
5. any more suggestions of places that i can swap or places that i should stay more or less days will be really appreciate it.