G'day @Ishtarduzzie
I will cover a few of your questions there.
First up a two stage sediment filter is not a RO unit. It will not remove the salts and chemicals that are present in your tap water so you will still have to use dechlorinators and the PH/TDS of the water will be the same. Google Reverse osmosis units and you will see why. A remineraliser may not be needed.
Not all plants need a substrate, just the root feeders, but all plants need some fertilizer. With a thin layer of iron sand there might be enough flow of water through the sand that even root feeders could get what they need if the water column is dosed. Most shrimp keepers use a buffering sub to bring down the PH of the water to something close to what the shrimp prefer not really for the plant growing ability. Your shrimp selection, tap water/RO, plant selection will dictate what sub to go with. Going with iron sand, that is inert, will mean getting the water to the desired parameters before adding and having nothing in the tank that will affect it.
That being said buying your shrimp from a local breeder with the same water as yours can negate all the hassle as a lot of the shrimp out there have the amazing ability to adapt to the local conditions and will thrive with what you already have.
Disclaimer: This is advice and it's free. You get what you pay for and i'm taking no responsibility if you put $200 worth of CRS in tap water because the breeder said it was ok and they turn turtle.
I hope some of that helps.