We have a 10,000 litre tank and a 2500 litre tank that we collect water in. in our situation we are supposed to in a good year have six months of rain and six months of dry season. Last year was and exception. almost 8 months of dry and rain was sparse in rainy season. This year has so far been better for rain. We have had 7 months of rain so collect every precious drop. You are absolutely right when you say municipal water kills healthy bacteria. The grey water does smell awful. I am glad we don't have to worry about the water freezing. That's a whole other ballgame and you seem to have that well in hand.
Do you filter the water as well? Similar to what we are doing or different? Thank you for confirming much of what I spoke about here. Yes, the extreme cold is a challenge, but we have it all worked out. We get by with up to 6 months of freezing temperatures. With only 88 frost free days per year, it can get a bit touch and go, but it works.
We use a very similar method to yours for filtering. But I have thought in the future of buying a Berkey water filter to do a further filtration. We do have times when there may be contaminants settling on the roof. We have never had our water tested and it might be worth while to have it done.
I have an old pressure cooker and I played around with making distilled water from it. Worked well. So if push came to shove, I could distill water without much difficulty. Especially for drinking and cooking, which does not take much water. It could sit on the wood stove and distill all day long. Just need to keep the pot topped up once in a while so it does not boil dry.