How can I easily obtain a COVID extension for my tourist visa in Koh Samui?

Samui and KPNG are probably the easier options. They were giving the 60 days straight up ( no consideration period) I don't know if that has changed though.

Any of the major immigration offices are usually easy to obtain. Some smaller offices will outright refuse or give you major hassles. Sorry I have no experience of the Samui office or their policies.

I like your question and thinking. Cause indeed, the requirements for the same outcome can be very different throughout Thailand and whatever immigration office you use. That is deeply flawed and unfortunate, but an actual truth. Unfortunately, there isn't an absolute answer to that, as particular requirements and procedures tend to change even at the same offices. So all I can advice is: 1) Start checking at the immigration office responsible to where you are staying now. And IF they make it exta and too complicated, move to another amphoe / province, stay a night and shop for more reasonable circumstances. It is in my view one of the biggest issues in Thailand, that they interpret feudalism in a way that every little sheriff can change anything to their liking, even though the underlying law and regulations of a certain visa or extension is the same. Just something we need to deal with - or better yet, to circumvent by choosing the more advantageous place.