My main concern is buying a ton of stuff that are duplicates and end up overdosing on something - potassium seems to be in lots of these.
Water is very hard in my area.
306.5calcium carbonate (mg/l)
I dont need anything to boost my GH
Since you have hard water(lots of calcium and magnesium in your water) you most likely don't have to add anymore calcium
I also have quite high nitrates present in my tap water @40ppm
KN03 has potassium but also nitrates
KH2P04 has potassium and phosphate
K2S04 potassium sulphate
K2S03 potassium carbonate
Since you mentioned you have a lot of nitrate in you tap water already you may only need to add phosphate and Potassium.
KH2P04 has potassium and phosphate is a good way of adding these.
KCL is another way to add just potassium
Fleet Saline Enema is another way to add just phosphates.
Also, it's very hard to overdose potassium, almost impossible
You'll may still have to add micro-nutrients like Flourish Comprehensive or CSM+B.
And of course you can't forget the most important plant nutrient Carbon (CO2)