I have spent time (almost 15 years) in Thailand on various visa's including Education, stamp on entry, and the last two on a Retirement Visa. I have never had a problem with any of these and although I was apprehensive about border runs and renewals these all went very smoothly, we just tend to worry too much about things. With your record of not being in the country for years on end then I honestly don't think any Immigration Officer would even think twice about putting another stamp in your passport and once you are on retirement then you have zero issues. But for peace of mind go and see a Visa Agent and you will get sorted with no worries.