Shrimp substrates can be used with tap water, it is how i started. Using tap water will shorten the lifespan of the substrate, but not as short as you are suggesting. They will last around 12 months with tap water, 2-3 years with RO water (from my experience) This does not include fluval stratum though, the stuff is useless with or without tap water. Akadama is good but the diarrhea colour always put me off! It is good at buffering the water but last no longer than a shrimp substrate, so its best to use whatever you can get cheapest.
Cattapa leaves can lower the ph but only if you use large amounts of them, its not good to rely on them to buffer the water. They are best used for their tannins and medicinal qualities. There are products that can buffer the ph, Soft Water, Fulvic Grain, Humic etc but you can't beat a good substrate. It keeps you ph below 7, kh at 0 and gh at a good level, its expensive but not as expensive as replacing shrimp when they die due to wrong water conditions.