“I wonder how rainwater (naturally soft) may react with glyphosate?”
(My initial web search did not turn up much.)
Soft rain water and near neutral ph would be as near perfect as one could get. When I was farming we got good results with even our very hard water. Just don't forget the surfactant (soap). We used Amway SA8 by the barrel. It doesn't take much, we used a pint per 300 gal so a tablespoon in a hand sprayer is plenty. It helps the chemical stick to the leaves and cuts through their waxy coating. Put it in last so as not to get a lot of foam.
I am not a soil scientist, just a forgetful retired farmer so treat the free advice for what it's worth.
WATER HARDNESS AND pH IN THE EFFECTIVENESS OF GLYPHOSATE FORMULATIONS