The difference in nutrient export between 5 20% changes and one 100% change is about 71% efficiency for the 5 changes , ie about 71% of the old water will be removed with 5 20% changes as opposed to 1 100% change. The nutrient removal totals will be higher than the 71% as more nutrients will be added to the tank over the time elapsed during multiple changes.
Nobody can truly say what's going to end up in your bucket.
I agree that salt mixes are harsh and unpredictable particulary for trace elements,ammonia, amines and especially free metals . This is one reason organics are added to some salt mixes to bind impurities and alleviate toxicity but those organics take sometime to bind them. . All salt will bring in metals higher than nsw values accoridngto several studies . Most tanks have high metal content. Fortunately , 99% of the metals in our tanks are bound to organics which makes them bioavalibe and reduces toxicity as organically bound metals don't pass trough cell membranes very well.
Personally. I would not do large water changes except in an emergency and then I would age the new water with aeration for a mix containing organics as much as possible.
Water changes put a dent in nutrients but are not very good at managing them without getting at he underlying imbalance between import and export. There are better ways to manage nutrients even in high biolaod systems without shocking the system via large water changes,imo.
Water changes are important to help alk,calcium and mag as well as to replenish minor and trace elements and to reset elememtal ratios like sulfate and chloride. Regular replensihment with small changes clearly sustains higher stability in the overall water chemsitry which is a good thing for the animals, imo. It also cuts the risk of toxing a sytem ,errors in mixing or otherwise doing harm with larger voume changes.