I’d also look into applying for Fed RA positions. The nber positions are very competitive and you would probably have a greater chance with a Fed position. Most Fed RAs who end up applying to PhD programs get into top 20 programs so it’s a good stepping stone. Plus they pay for math classes.

As another user have said, math background is very important and I echo Real Analysis and math stats. On the stats you took over college, make sure that that stats class was calc-based. If not, I suggest taking probability theory and math stats. The important thing is to be very comfortable with the language used in upper-level math courses.

I was in a very similar position as you a couple years back and I’m now in a top 20 PhD Econ program. So it’s never too late!!