I agree… Since the TM6 arrival/departure card is no longer in place, the boarding pass has all your details about you that the airlines already obtained. The TM6 only ask for lodging information which has very little impact.
Just remember not to keep your U.S. Bank account(s) dormant. To keep it active, make a small deposit at least once a year. Some banks will deem an account inactive if no activity is made within a 1 or 2 year period and will attempt to contact the owner using a mailing address on file. If unable to contact the owner, it will be sent to the owners state treasury for safe keeping until it is claimed by the owner or heirs.
If you do purchase a condo, make certain the building is not a lease-hold property but a free hold property. Meaning you own the condo but lease the building usually 30 years and after 30 years, you have to lease it again.
Note: Once you exit Thailand, that VISA stamp you entered with is no longer valid.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.