retain your spent boarding pass in to Thailand as proof of incoming flight and hand that over to the Immigration office with your passport. Then have your flight out booking confirmation printed to hand over as well. The 60 day visa exempt is a very simple, no fuss way to enter the country. You will have no issues my friend and enjoy Thailand.
I think you were just unlucky on the day and got an immigration officer in a bad mood. You will always get more hassle when your passport is full of stamps. Just the way it is. It seems you are not breaking rules, or even abusing the system like some people who have back to back extensions are. I would just forget about it and carry on as you are. To make life easier, I would probably book a very cheap flight out of Thailand within the 30 days to prove your intentions. Yes, this might be wasted but this is actually a requirement anyway.
P.s. your written English is great, so I very much doubt spoken English was a factor here, especially when most IOs donβt speak much English themselves.