How difficult it is to pursue research in a field that you are not specialised in? I will be finishing my Bachelor's in Economics soon, which I regret, because my main interest lies in rigorous mathematics (which Economists only use because it feeds their ego, not because it is particularly useful).

My main dilemma is whether I should continue with this shithole subject and pursue Master's and consequently Ph D, or should I try switch to Mathematics (something that will be extremely difficult since I am not prepared for the Math entrance exams).

Economics is not completely out of my interest since there is a lot rigorous math sometimes, and I am fine with teaching it, but I am wondering if I could get pure math and statistics research opportunities with an Economics Ph D.

Do note, I am trying to get into the absolute top Universities in my country (it is free and I get a small stipend). If I complete my masters here, I can definitely get an Econ Ph D opportunity maybe even full-ride in something like Harvard.

Since I am not allowed to sit for the entrance exam for different subjects in same year in same university, trying to get a Math masters is going to severely lower my chances of trying to get a Econ masters. Should I just play it safe and go with Econ?