Если я вернулся из бордеррана, надо ли опять оформлять регистрацию в миграционке?

Кажется, с 2020 года это не требуется. "In the event that a foreign national travels and stays in another place on an occasional basis, and then returns to stay at the same premises within the period of stay specified in the T. M. 30, then a new T. M. 30 will need not be submitted each time they arrive. The new rules are applicable to all foreign nationals who stay in Thailand on any type of visa, including holders of multiple-entry visas and re-entry permits which enable the holders to leave and return to Thailand within their visa validity period."

Ага.