So the worst that could happen is some iron in my nute solution?
I have no worries about that.
I have been using them for well over a decade now with no issues. Using a 3-year-old set of pumps I recently got 1.3GPW from HPS. I do recall once using copper pipe to build a system which made my plants rapidly head in the direction of death because copper blocks iron absorption, isn’t brass an alloy of copper and zinc? That might be the issue you had.
I even started using old central heating pumps taken out of an old central heating system which were given to me by a plumber friend (although they were also decent Grundfos ones ). I have tried cheaper brands but some of them did not even last a single veg/flower cycle.
Whatever they do at the Grundfos factory, it is the right things.
Now I think, I have never seen a brass casing on a central heating pump in the UK.