Social impact consulting work is not necessarily pro bono. Though many consulting firms do some pro bono work, they need to pay the bills. Many clients of social impact consulting firms have grants or other sources of funding.

With that said, your idea of getting some social impact experience part-time is a great one, and I’m sure your software development skills would make your help highly valuable. Look for opportunities where you can take on a discrete project and quantify the impact of your work. That would allow you to get the most out of the experience when you write your Consulting Resume. See this guide for more details:
https://www.myconsultingoffer.org/cover-letter/consulting-resume/