Note: Once you exit Thailand, that VISA stamp you entered with is no longer valid. You can get another 60-day VISA exemption when you return. Once you return with your new 60-day stamp, you can extend for an additional 30 days at the Immigration Office to total 90 days.
As others said: use your USA passport to exit USA, enter Thailand using Thai passport (stay as long as you wish). Exit Thailand with Thai passport and enter USA with USA passport. Very likely airline gate agent may question if you have no return ticket. Just show your Thai passport and they will note it into their system. We do it all the time.
When going to Laos or Cambodia, use your Thai passport as no visa is required for Thai passport holders.
When departing Thailand, use your Thai passport to exit Thailand immigration. At the departure airline, the gate agent may ask to see your USA passport.
Upon arrival to USA, Immigration officers or kiosk in USA will just welcome you back. Questions they will ask: are you bringing in food, how was your trip, etc.
These are the countries eligible for the 60 day visa exemption. If your country is not listed, you don’t get the 60 day stamp. If you are listed and got a 60 day stamp, then it was in error and should be corrected immediately while at the airport.
Also got the offer from EVA with the minimum bar set at $1600 USD but it was only for one segment (flight from TPE to ORD). BKK to TPE (Premium) didn't change. I opted not to play the slider game.
Reading news publication is only a teaser to get you interested in the story. It becomes law once published into the “Royal Gazette” with the effective dates.