What I meant by fauna in my last post is primarily bacteria. It's entirely possible mineral content plays a role as well (GH, KH, and TDS are limited in what they measure).
Out of the tap my water has a GH (calcium hardness) of about 50 ppm and a KH (carbonate hardness) of about 70 ppm - this comes from a deep, untreated community well. The PH is generally around 7.6 - 7.8, even with the low KH that I have.
I use that water unaltered in many of my tanks (including my thriving malawa shrimp and blue velvet shrimp tanks), but I buffer it with Seachem Alkaline Buffer (primarily carbonates) which pushes my KH somewhere over 100 ppm so I can avoid PH crashes. With my KH as low as it is if I overfeed or use soil as a tank substrate I end up with a PH that dives through the floor - neocaridina and the fish I keep don't like this very much.
Temperature varies from the low 70s to the upper 70s in most of my tanks, though I do keep RCS in an unheated tank that's usually round 65.
All of my tanks are planted, some heavily, and I use dry ferts (plantex csm+b, potassium nitrate, and monopotassium phosphate) in many of them plus flourish excel (though I avoid this on my dirt tanks and some of my shrimp tanks; my RCS and malawa aren't bothered by it).
Not all of my shrimp tanks are successful, unfortunately. I'm watching my blue bees and yellows slowly die for unknown reasons in one of my tanks. I'm guessing there's something in that tank that I don't have in any of the others that the shrimp just don't like. Like your tank, all the parameters I can measure are fine and some shrimp are okay, but others are dropping off one by one.
I'm glad I helped suck you into shrimp and planted tanks
I'm not so glad I helped suck you into the money pit that is dead shrimp... I've wasted hundreds of dollars on shrimp and crays that didn't make it, but there are others that have absolutely thrived for me. The hobby definitely has its ups and downs.
One interesting anecdote is that tank 5 (the infamous tank 5) killed at least 5 CPOs (dwarf orange crays - not particularly cheap), many more dwarf cajun crays, and my yellow shrimp never would breed in it. I moved the yellows to tank 7 and they thrived (until the CPDs ate all the babies...oops). Tank 5 was overrun by malawa shrimp and male endlers after that, with some CPDs in between. Now tank 5 is home to a thriving Blue Velvet Shrimp and assassin snail population along with a ton of algae. In that time (2+ years) I vacuumed the substrate twice. What changed!? Who knows.