Yeah cast Iron is not as bad as brass it seems, and might work fine when you just change the pump once it’s been eaten.
https://www.hindawi.com/journals/ijc/2011/506501/tab3/
Most of the minerals it will leach, will be eaten by the plants.
It’s micro nutes, Mn, Mo and S that worry me. But if you have been running hydro with these, and don’t see issues with odd difficiencys and/or over feeding. Im guessing it’s not as big a deal as I assumed, it might be.