For the past several years I have been using Asus routers running Tomato firmware to do exactly what you describe. I didn't know the stock firmware supported OpenVPN. But Tomato might be a happy middle ground for you - Linux based but much more user friendly and tolerant of user error than DD-WRT.
Check out some of the older Asus models - RT-n16 if you don't need 5GHz or RT-n66u if you do.