I have a METV with a 90 day validity from day of issue. So I must leave Thailand every 60 days - at least.
BUT, when the validity is running out, would it be possible to extend a 60 day period with 30 days (the 1900 bt procedure), to a avoid the trip out, but also consequently making the visa null and void?
TLDR : Answer Summary
A Multiple Entry Tourist Visa (METV) allows for a stay of 60 days per entry, which can be extended for an additional 30 days at a local Thai immigration office for 1900 baht. It's not necessary to leave Thailand every 60 days, as one can apply for the extension instead. The extension will not invalidate the METV, and if you leave and re-enter Thailand near the expiration date of the visa, you can receive another 60-day stamp, which can also be extended for 30 days, potentially maximizing your stay significantly.