How can I get feedback for my work in math if I'm not in academia?
Nowadays, you can get feedback and answers to questions from places like MathStackExchange, and MathOverflow. Most of the responses are (in my observation) either entirely competent or quickly corrected by others. Good quality control. No, that wouldn't be the same as having a good advisor (ideally a world-class scholar on your subject) that you talk to in person every week. Still, these sites can give guidance.

Also, quite a few of the more-senior people who like to contribute to such sites may be amenable to direct email contact, etc.

(Grad school is not about "taking notes in lectures" per se, but is about having access to the informal observations of very good mathematicians, and being able to ask them questions immediately, in the moment.)