Carbon doesn't last long in my filters, I guess just from water eroding it?
I do not use it anymore, because anything that can be removed with carbon can also be removed by water change.
As far as nitrates being good or bad for shrimp, it's not the same as when you add nitrates as a fertilizer. When nitrates are present in high amounts in an aquarium that DOES NOT get fertilizers added to it, indicates a lot of decay, organic matter, phosphorus, etc that ARE harmful. High nitrates are simply an indicator that something is decomposing in there, and that you need to do a water change.
So instead of using plants to absorb lots of nitrates, try doing water changes instead.