A small update, I am changing pumps and fittings to remove all metals in contact with my nute solution. Turns out you were more on the money than me and because of a situation last year where I had PH 1.5-2 solution pumping round for a few hours corrosion started in my fittings and pumps. This did indeed lead to an S def from copper interaction AKAIK. I have replaced them like for like but I will be replacing them again very soon with plastic fittings.
I had a nice surprise in getting an Iwaki MD55 pump for under 1/5th of the retail price which has only had about 100 hours running time.
I have decided it is far better to not need a replacement schedule.