Depends on the IO. I entered Thailand probably 10 or 11 times visa exempt last year but majority of my trips were for 2 nights with longest being for 2 weeks.
If you're here for 30-90 days per trip then you have a higher chance of being flagged as they may suspect you of living and/or working in Thailand.
single entry evisa for VN is USD25. Cambodia evisa in advance is more expensive than Vietnam. Not sure what Cambodia's fee is on arrival but I bet it's not cheaper than USD 25
Sihanoukville is a shithole but it's where you need to go to get to Koh Rong, which is stunning. Kep is boring and the beach is crap, Kampot is a much better choice despite it being a riverside town and not coastal.