There is an ergonomic grip that you might try on your bike as it is. How long of rides do you go on? ERGON BIKE ERGONOMICS

I don't know how it would affect the thumb, but it might help with the hand in general.

The Trekking butterfly bars should give you a variety of grip positions, but allow you to reuse most of your MTB parts, making the conversion much simpler than doing a full drop-bar conversion.