There are definitely Thai Airways transit counters at BKK and you do not need to exit immigration if transfering to a Thai Airways flight. Being your flight is a code-share, not quite sure how that works. Best to call Air India to be sure if they have a counter there or if you can use the Thai Air counter. Most major airlines do have counters there, so my guess is the answer would be you can do it. It's normally only the discount air carriers that don't have transit counters airside before immigration.

I you can get your onward boarding pass airside, then there's no reason to pass through immigration and so no reason to worry about any sort of VOA or transit visa.

The Thai Airways transit counters are located at either end of concourse D.