Hi! I first must advise you that my B.S. wasn't in Math.

I know that there are good programs in financial economics like the Oxford's MFE but I'm not sure how quant it is. I know it's a top program with a quant side, but surely not as quant as the Master in Finance of Princeton (or any other top program as in here https://quantnet.com/mfe-programs-rankings/)

I think it's not worth going through another Bachelor's degree. If you can take separate quant courses in an university/college it may be enough to show the admissions committee that you had enough exposure (and did well) with quant subjects. But of course, you would need the credits for these courses and I don't know how it works where you live (UK?). If the univ. is free there, then it's a different context.

These are my two cents, I'm just another lost 23 y.o.