here's what the official Thai revenue website says C. Elimination of double taxation
The focus of a DTA is the elimination of double taxation. Each DTA may prescribe different methods of elimination of double taxation of a person by the resident country:
(1) Exemption method
The country of residence does not tax the income which according to the DTA is taxed in the source country.
If the money you bring onshore has already been taxed under a DTA, you do not have a tax liability in Thailand even though you are technically a tax resident.
The property owner is responsible to file a TM30 within 24 hours of an alien arriving. You only need to worry about the TM 30 if you are going to immigration for an extension