For dilution I would recommend RO/DI, as opposed to straight RO. The overall cost wouldn't be much higher, and the overall purity is higher. I'd recommend against storing RO or RO/DI for too long, unless you are meticulous in the cleanliness of your RO storage. Remember, RO or RO/DI no longer contains disinfectants, which may make it a nice place for bacteria to grow.
For mashing and fermentation, you might be able to get by with a newer nano filtration unit. Less pure than RO, but significantly less waste too. And based on your source water tests, might save money on adding salts/minerals to RO.
Highly recommend reading:
Water: A Comprehensive Guide for Brewers
by John Palmer and Colin Kaminski
Part of the same series as White's Yeast book above, a bit harder to read if you slept through chemistry though.