Dirk, I would like to hear about you adventures when you have time.
Here's my thread about the water softener and the rain water, along with a sort of journal of how I learned a lot of what you're learning in your thread:
Water softener connected to auto fill, and new plaster start up.
The short-ish version:
Your fill CH is about the same as mine. Only pure water evaporates from your pool, so the CH that's already in your pool stays there. As you refill the water loss with your CH-rich water, your adding more CH to your pool, which also doesn't evaporate. So by doing nothing more than keeping your pool full of water, your CH is rising. Even if your fill water had only CH50, this accumulating would still take place (it would just take longer). So there's only two solutions to this problem: replace your high-CH pool water with lower-CH water, or never add any CH and maintain the level of CH that's already in there.
Rain water has virtually no CH in it. So what I did was to drain off an inch or two of my pool just before each coming rain, and let the rain fill the pool back up. This, in essence, is like draining your pool to replace the CH-high water with new fill water and starting over, but using CH-zero rain water instead of CH325 street water, and doing so an inch or two at a time. I increased the efficiency of this process by collecting rain from off the roof and diverting that into the pool along with the rain that was falling directly in. You can read in my thread how I did that and what the results were. It works, but requires some effort, and only does any good during the rainy season, which in my area is not so much!
The better way for me was to stop putting any CH into the water, so that I wouldn't have to replace any of it in the future to bring down CH, with rain water or otherwise. I did that by replumbing my autofill system to my water softener. My pool started off with CH350, from the original fill from the street. I actually lowered by CH to 325 using the rain water experiment. Since then, by replacing evaporation with the CH-zero water from my water softener, my CH has remained at 325 for months. Theoretically, if this keeps up, I'll never have to deal with the issues associated with high CH (like scaling), and I'll never have to replace my water due to "CH Creep."
I detail in my thread how I connected by pool to my softener, while also maintaining its connection to the original source of city water. This way I can alternate my fill water source from CH-zero water to CH-350 water, whenever I need.
Why you ask? Because if you have an active pool, with a lot of people splashing water out of it, and if you replace that water with CH-zero water (from rain or softener), eventually you'll see your CH level go down (CH doesn't leave your pool through evaporation, but it can leave your pool if people are splashing water out of it). And low CH is as bad for your pool as high CH (a little worse, actually). So you could add chemicals to address that. Or you could just top off your pool for a while with CH-high water, to restore a healthy level of CH for your pool, without having to buy any chemical to do so.
Let me know if you need any other details...
