Hey @marid I came from a similar professional background to you! I also worked at a nonprofit that provides community-based services for people with mental health conditions in my state, and recently accepted my offer for Columbia SIPA so that I can eventually advocate for the same population in a variety of policy areas.
It sounds like your academics, internships, and work experience are all very solid. The biggest question I'm wondering, and that I imagine the admissions folks would also wonder, is why go back to school for an MPA when you already have a degree in public administration? MPA programs, while extremely variable depending on the school you go to and your concentration, are usually a split between practical business/management/economics/finance classes and more theoretical policy courses, but I'd expect you would have already covered a lot of that as an undergrad. If you're looking to move towards research, policy analysis, or other forms of analytics in the field then it definitely makes sense to return to school- but my recommendation would be to think more seriously about an MPP or MPH.
So long as getting a higher degree really is an important step in meeting your goals and you have a solid answer for it within your SOP, you will be fine! Point to a specific problem you've encountered at your org and discuss how a higher degree, but especially that school's higher degree, will help you solve it.
The biggest weakness in your application will probably be your quant grades. However, I definitely recommend explaining the lower ones in your supplemental essays, and those grades could be made null if you get a high score on your GRE Quant, so devote some significant time to studying for it.
