Interesting. New GH test is on its way. It's entirely possible (or even probable) that I'm overthinking this, and that a simple low KH has been the primary cause of invert death. In which case, weekly baking soda should be an easy fix.
I really don't mind the high GH. Outside of a handful of soft-water fish and plants, it's not really a limiting factor. But it would be nice to know what minerals/metals might leeching into the water from pipes. My neighborhood is 60 years old, so pipes are suspect.
I switched to RODI water as we have 50-60 year old pipes as well, but I did it primarily due to wild swings in cleanliness more than parameters, one minute it will be clear, the next fairly brown from rust breaking loose. I suspect those events also come with some changes in parameters as well. But I doubt that I NEEDED to do anything at all, I am just a bit OCD. Most people around here keep them in tap water with whatever comes out, and I never hear (we have a local aquarium club) anyone complaining it causes problems. Indeed, one guy had discus swimming in roughly PH 8.0, maybe a bit higher, and looked healthy and well. I really think people fixate too much on the "prefers" parameters for fish, when all the really usually need is time to adjust and a stable environment.
That said, I've had lousy luck with shrimp, period. For some people they grow and breed almost uncontrollably, but I've gone through two attempts with store bought, and two with wild caught ghost shrimp. I've tried various recommended GH/KH and it just didn't seem to matter. I now just buy my shrimp frozen.
Could it be too-low KH? Sure. But I bet it isn't. Especially with tap water, as it's almost universally kept a bit on the alkaline side. And you said 2-3 degrees, which seems reasonable anyway.
It's also possible it is something unrelated. Many areas use chloramines now not sodium hypochlorite. The latter is easy to neutralize with almost any conditioner, the former harder (Prime is reported to work well) and it also lasts longer in standing water, and goes through many carbon filters. Maybe there's something unrelated to GH/KH in your water that is bad for your inverts?
Are your inverts in a separate tank? Small tank? If so, one way to just eliminate it all is switch to RODI water for a while, mix to your liking (any specific KH/DH you just mix up) and see how it goes. You can buy distilled in a store or RODI from most LFS cheaply if it's a small tank. Not practical if it's a large one of course you would need your own system.
Or... maybe you are just like me. Cursed to have inverts die without obvious cause. In which case at least there are two of us.