Fran, I think you should aim for at least 3 Gdh KH just to be safer from large changes. As Loachloach said, there are always holidays or illnesses that can interfere with the best water change schedule, better to have some leeway.
My husband is a medicinal chemist (30+ years) & absolutely hates when I ask fish tank water questions, lol. He explains & draws structures, (but there's more to our tanks than lab test tube experiments, my take). He pretty much agreed with Bas Pels from his very quick glance...but did say "carbonic acid" is a false idea, it's not an stable compound even in the shortest of terms, it doesn't actually exist (my words, not his)....Then he segues into if this was a (pick an element
) instead of sodium, it would be this (more structures
) that breaks down into...goodness knows what
As far as CO2 toxicity goes, I thought Tom Barr did real tank comparisons & it's more a lack of sufficient oxygen rather than just high CO2 level that is problematic to fish...It "was" thought at higher co2 there was less oxygen in the water, but that seems not to be the case, they're not reciprocal numbers. He advocates surface movement for oxygen rather than atmospheric, or added co2, off-gassing co2 is not so much an issue. Now, I'm just paraphrasing as I understood it. I'm only a casual reader of adding co2 & all it's issues. I haven't tried adding co2, yet, although I might someday but not with fish at first.
I'm all for tank surface movement, most loaches like a good oxygen level, & we all here love loaches