No worries, happy I could help.
There's plenty of potential problems with tapwater. As you correctly guessed, regardless of how soft it is (unless 0), over time replacing evaporative loss the hardness will climb. There's also high potential for unacceptable copper or other levels, some not testable. Don't even consider tapwater unless the GH is virtually 0 in your area.
The poor man's alternative to RO is rainwater. A brita filter should be ok, essentially I'm using the same - a poly prefilter followed by a carbon core in 2 standard 10" housings. I use a big pond pump to force the rainwater through the filters or run tapwater through them at mains pressure. Beware that some carbon filters really spike the pH which means you then have to store that water long enough to bring it back down before use. A cheap RO unit is about $100, not much more than my setup and a much better idea. Squiggle has posted a link to a good RO unit supplier a couple of times. Sorry I don't have that link handy.
When I first kept CRS they were kept in a 3' tank with laterite substrate under normal gravel, lots of plants and an air-driven sponge, tap water changes. No problems. That population exploded quickly but also crashed again when I began to neglect the fishroom due to personal issues. Nonetheless I had CRS in such conditions for about 4 years. So it can be done simply but I don't recommend it. These days we can do much better.
As Disciple says, over-filtration is good. If a canister is truly out of the question then definitely add an air-driven sponge or 2. The shrimp love to graze them anyway. You'll need a sponge on the HOB intake too. When you read through Squggle's journal as recommended above, you'll not the spare changeover sponges in the tanks. Good idea and more passive filtration area. The substrate itself will help too. You could buy some low density marine pure - the rocks, cubes or rubble and have some just sitting in the tank. That will help a lot. Check out our sponsor The Tech Den for these. They're a great supplier.
The Benibachi substrate is reported to last around 2 years which seems as good as it gets from these active substrates. They lose their buffering ability over time and eventually start to break down to powder. I think the Cal Aqua might physically hold up longer but that remains to be seen. My advice is not to rely on their buffering ability and control the water parameters yourself. The Cal Aqua is designed for plant growth but works well for shrimp. The Benibachi is designed for shrimp and as I said before, is the choice of our best breeders.
Read through the journals that Disciple recommended, check out ShrimpyDaddy and see how you go. We're all here if you have more questions after that.