Multiple filters or not?
Your goal should be achievable. The key driver for the filter's complexity is the size of the transition band, which you haven't specified. If you can live with a slow rolloff, then you can use a relatively low order filter. 

If, as is more likely, you want to isolate the band from 5 to 50 Hz with relatively sharp cutoffs, then as you have started to discover, you will want to use a multi-stage approach. Instead of decimating by a factor of several hundred all at once, you decimate by a series small factors (something like 2, 4, or 8). By gradually decreasing the sample rate, you end up with filter designs that are much less complex.

This previous question has a very similar aim to yours, only with slightly different filter specs and sample rate. You should be able to adapt the information there to your application.