I live near a small Mexican border town. I travel to Mexico frequently through this town, often for weeks or months at a time. The only documentation I ever show is my driver's license and auto registration before I'm waived through. Upon my return I show my passport card. It gets scanned but there is no way US border patrol have any idea how long I've been in Mexico.
You're correct, but the METV permits the holder unlimited entries into Thailand for up to a period of 180 days in total with required exits every 60 days. Whether one extends one or more entries is immaterial.
For a bit of clarity, there is the SETV (Single Entry Tourist Visa) a e-visa issued for 60 days. There is also the METV (Multiple Entry Tourist Visa). Which allows up to 180 days stay. It appears the METV is what the OP obtained.
Do mind that it has a bit of an expensive application fee but once issued there will be no need for extensions or reentry permits, which can add up. Good luck.
I've been using the METV for several years running, (typically for 5 month stays). It allows for 180 days with two mandatory exits after 60 days. If you actually plan a 6 month stay and take two exits to neighboring countries there'll be no issue with staying under the 180 day threshold if that's a concern.
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The METV (Multiple Entry Tourist Visa) allows for a stay of 180 days and costs $200 USD. You are required to exit Thailand at the first and second 60 days but otherwise need never bother with a visit to immigration. Frankly you can exit/reenter as many times as you wish on this visa whether by air or land. Others will chime in with advice for how to stay for as long as 9 months on an METV. Your use case is similar to that of my wife and me and it's proven to be hassle-free and economical for us.