Plastic tanks are not used because they leach chemicals into the water.
I am curious about the evidence of this comment, in Australia there are thousands of plastic tanks installed , all with certification about their water quality standards.
Rumours always travel faster than the truth.
Multiple filters will reduce the water flow.
In Australia we do not use them, rainwater is ok without them, if the storage tank is at least 20,000L in volume.
But if you increased the size [ surafe area] of the filters, by using a larger one, or using multiple ones in paraell, the resistance will be reduced.
The use of a tank in the lines will not change the flow issue substantially if its place prior to the filters.