If you leave the country and come back, your IDP is good for another 90 days. Also, the IDP expires in one year. This should give you time to get a Thai drivers license, which is simpler with an IDP.
The six-month validity stamp in your new passport likely reflects the current entry period allowed under your DTV visa (180 days per entry) rather than the full five-year validity of the visa itself. This is standard for DTV holders, as each entry allows up to 180 days of stay, after which you must either extend or leave and re-enter Thailand.
You’re not supposed to apply more than 3 months from expected entry date, but in actuality you can come and go when you please during the five year term.