Unfortunately the local fishkeepers have misled you, and that is easily done as water providers (and even aquarium test kits!) have differing definitions of soft/hard to the definitions used in fishkeeping. So the values given in post number #19 are what you need, giving you Soft water at 106ppm/6°GH- these are the 2 measurements used in fishkeeping, ppm and German degrees hardness.
Agreed. This can lead to swinging parameters which are detrimental, even deadly for fish. The GH, pH and KH are intrinsically linked and complex, there is not a simple recipe to move one aspect without effecting the others.
This is one of the most frustrating aspects of the hobby, especially for new fish keepers. Information is conflicting and confusing. There are many unreliable websites and you tubers, and from your examples, even books. I like the website
https://seriouslyfish.com/ as it has a scientific knowledge base. Then, of course, there is the advantage of having just one reference to simplify things!
Another way of looking at it is safest to go for the mid-range parameter number (Seriouslyfish.com always gives a range, not a specific number) so that the fish are not exposed to the extreme limits of their comfort zones.
After all, there are so many things that can effect a fish in terms of stress and health, why add other variables into the mix when we know better?