This is my advice, so as with any advice, take what you think is relevant beneficial for your case. A lot of people in the states up their math at community colleges (a cheap way to get mostly good quality math experience). There is also a rise in pre-doctorates which are a bit different from a master since is mostly research/networking rather than standard coursework, but most give math courses. Now, in your case you already have a good math background so it should be relatively easy to pick up math used by economists. In terms of MOOC, I would focus on programming/data science ones. I was addicted to Coursera for the first couple years of my program and learning those skills have opened a lot of opportunities for research, internships, jobs, and currently for post-docs.
TLDR: Math is math. Look into pre-docs as these are ever increasing. For MOOC, rather than math do programming/data science.
