I know of a few peers who ultimately working with environmental law and would say that in general, becoming an environmental lawyer is more of a specialization rather than an entirely independent field of practice. For example, environmental issues come up all the time in real estate transactions and one portion of the work involved determining if it's safe to build residential property in an area where, say, there used to be a chemical plant or dumping ground in the distant past. I also know someone who became a "water" lawyer handling farmer river usage rights, which people take a lot more seriously than you'd expect.
Regardless of the specific subset of environmental law, knowing how to draft letters and review documents is a staple part of lawyering and having those skills early is a great boon.
I would say there are two paths to policy making. With enough time and experience in the field, the prospect of being in the position to take on a novel matter comes up from time to time and this could be a place where you could change existing policy. The other path would be to work towards becoming a legislative assistant in the state you want to practice in.
Hope that helps,