I'm kind of similar to you in the sense that my undergrad was also in economics & math while my masters is in pure math / stats. I'm trying the opposite route though where I'm going back to economics for my PhD.
Heres what I can tell you after talking with my stats profs/advisors about grad school for the last 2 years.
1) Taking more courses in stats/prob at this point wont make or break your application so really take whatever interests you. As long as your undergrad/MA gpa is decent and there are no red flags in serious courses, the marginal benefit of taking more courses from a signaling perspective at this point is very limited.
2) An RA job in the biostatistics department is great if you can get it but I dont think it would help you much in the application process unless you really stand out to your supervisor in the next 4-5 months. So get the job if you can and take a call after a few months of work if you want to use him as a reference or not.
3) If possible, dont use all economist references especially if you plan to apply for biostats. Try to get some references from people in stats , maybe from the professor you took graduate math stats with for example.