I had a similar thought back in 2014. I had only studied the maths required for various engineering courses I’d taken.

So, I decided I wanted to study maths for the maths. I was in the fortunate position of being able to self fund myself through the Open University (uk based) Maths and Statistics BSc. One module at a time I’m now on my last module. There many things I’d studied before (calculus, sequences) and many new to me (group theory, graph theory)