OK, I think that’s probably fine as long as you are very clear on the scope and the fact that some cause areas that you ‘funnel out’ may in fact still be very important through other lenses.
It sounds like you might be doing something quite similar to Charity Entrepreneurship so you may (or may not) want to collaborate with them in some way. At the very least they might be interested in the outcome of your research.
Speaking of CE, they are looking to incubate a non-profit that will work full-time on evaluating cause areas. I actually think it might be good if you have a somewhat narrow focus, because I’d imagine their organisation will inevitably end up taking quite a wide focus.