All 3 cellular carriers have booths at BKK. There is no price difference between company stores and independent resellers (7-11) as all the available plans are published.
Go to any booth after you clear immigration and hand the attendant your phone. They will have you choose a plan from the chart. Show your passport. They will load and test your phone to ensure it works.
This is not difficult.
Most will use an eSIM.
Some will add the Thai SIM to their existing as a second SIM and toggle back and forth using the phone’s settings screen.
I would never trust a 7-Eleven clerk to load a Sim into my phone however, the people that work in the airport can do it blindfolded, all day long every day.