With the METV visa, you still get another 60 days + 30 so you can extend the 6-month validity to nearly 9 months. So I expect it will be the same for a DTV, another 180 days and possibly + 180 which would give you nearly 6 years.
I always recommend using an agency to do a border bounce as they have agreements that help you get back in without issues. I would leave it as long as you can and try one more time to get back in via Nong Khai before you go to the expense of a flight to Bangkok.
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.