I’m 99% sure you can’t lower it because your PO4 is super low. It sounds counterintuitive, but dose phosphates, and the nopox will then lower the nitrates super fast. That said, if you manage to get the PO4 up to around 0.15-0.2, chances are your tank will look way too healthy for you to want to lower the values ever again
I agree, it sounds like you may be PO4 limited. I would dose PO4, and then let your chaeto take care of the NO3. Your chaeto should rebound with the PO4 dosing.