It might be longer than 15 days since you have to include Thai and USA holidays into the equation and the 15 days are working days so it’s actually 23-25 days since sat and sun don’t count
Why not just come over get a 60 day visa exemption stamp, then go to the immigration office pay ฿1,900 Thai baht for a 30 day extension….
After that do a border bounce either by land or air and get another 60 day visa exemption stamp which you can also extend for 30 day at the immigration office for ฿1,900 Thai baht