Would this work?

Planning a 3 month trip this summer in thailand, never been before. Original plan was to do a tourist visa + 30day extension but now I want to do a 30 day visa exemption, with a 30 day extension at immigration, and then do a border run at cambodia for my last 30 days. Would I have trouble getting into thailand with no visa? I’m from USA. Also, where in cambodia would be best for me to do the border run, if ill be staying in Rangsit? I’m aware poipet is shit.