Based on 6 experiences
I just returned after 2.5 months in Southeast Asia (Thailand -> Cambodia -> Vietnam -> Bali -> Thailand), which is practically what you are doing. I can initially say that it was absolutely amazing and I felt very safe the whole time (an 18-year-old solo female), although I spent a lot of time on organised group tours, which greatly facilitated the entire experience.
Posted: November 12, 2022based on 6 experiences
I spent a solitary 4.5 months traveling around Southeast Asia and visited Thailand, Laos, Vietnam, Cambodia, and Bali, and I enjoyed them all. I would definitely recommend Cambodia. Especially the Koh Rong islands - they are geographically close to the popular Thai islands, but they are much less spoiled and amazing thanks to sections of white beaches and crystal blue ocean!
Posted: November 19, 2022I loved Vietnam (my favorites are the Mekong Delta, Hoi An and Ha Long Bay (you can live on the islands there)) and Sumatra (check out Lake Maninjau and the orangutan sanctuary in Bukit Lawang). Don't book too far in advance, that way you can stay longer in the places you will love and vice versa.
Posted: November 11, 2022