>>14073650I think the problem is that math didn't actually advance all that much until the era of mass Western education. It did to be sure, but intellectuals were rarely focussed solely on math. Even Newton and Leibniz focussed more on Physics and Philosophy respectively. Speaking of Leibniz I guess George Boole is another candidate that might count, although he spent some time at university. I had thought that some early topologists were also academic outsiders, but having looked up the researchers I had in mind I was mistaken.
So basically yeah Heaviside is the only one I can think of strictly fitting OP's criteria. Math is just too difficult of a field to survive as a self-taught outsider I suppose. Most poor people are forced into engineering and rich people have always had lower hanging fruit in other fields.