visa waiver is OK,
B1/B2 visa is OK,
no TSA required as you are already a FAA commercial pilot.
it is just a small upgrading and you know already to fly for the FAA.
Now, if you do a rating over 12500lbs, you need TSA approval.
enter as a visitor, go in a small airport, find a freelance cfi and rent an aircraft, and pass your ATP checkride.
Good luck!