This is a cool thing to do. I followed a slightly similar path in that my undergraduate degree did biochemistry up to 3rd year, but at the expense of third year pure math courses. (I had a kind of applied PDEs and modelling course but applied to biological problems like gene flow, disease modelling and population modelling.)
I ended up doing honours in bioinformatics, but then took a year to do third year pure math (and the honours course in lattice theory and topology). After this I ended up doing what I really wanted to, which was an MSc in pure mathematics.
I also did probably the first 1/3 of Strang's course (even though I had already done linear algebra formally in my original course). I think online math courses are a mixed bag and it could even be that it works better when you have an idea already of why you want to do it and the applications thereof. Applications need not imply applied math—you can apply mathematics to other branches of mathematics.
The more difficult thing for me is to independently find research topics and subtopics. Mentorship in mathematics is very important.
I ended up doing honours in bioinformatics, but then took a year to do third year pure math (and the honours course in lattice theory and topology). After this I ended up doing what I really wanted to, which was an MSc in pure mathematics.
I also did probably the first 1/3 of Strang's course (even though I had already done linear algebra formally in my original course). I think online math courses are a mixed bag and it could even be that it works better when you have an idea already of why you want to do it and the applications thereof. Applications need not imply applied math—you can apply mathematics to other branches of mathematics.
The more difficult thing for me is to independently find research topics and subtopics. Mentorship in mathematics is very important.