Water chemistry is very complex and pH is one of many measurements possible. Because pH strips and pH meters are relatively inexpensive, people tend to focus on something easy to understand. But at the end of the day, it's only one measurement. Imagine trying to test food quality based solely on one measure, say saltiness. As an extreme example, pH 8.9 is hard water is very different than pH 8.9 in 100% R/O. (In fact, pH in R/O water is meaningless.) Not trying to frustrate you, but I've raised and bred discus at pH 8.7 - but my tap water is on the soft side (145 ppm TDS).
You can assess whether discus will do well in your tap water by checking out local fish stores with discus. They seldom have good quality specimen, but you can see if the fish are thriving: fins up, facing front, swimming mid-level, lively... Unless you're bringing in wild discus, tank raised fish are typically more adaptable. The key is not the actual water condition but clean water.
Discus will grow up and spawn in hard water, except that the hatch rate can be very low. That said, I did precisely what you suggested. Once I have a mated pair, I would wait until the male start to clean the cone. (Not the female, which cleans often and not always before spawning.) I would then do a 100% water change with pure R/O. The fish usually spawn within 2 hours into a completely clean tank with very soft water. There's no "shock" going into pure R/O. Once the eggs hatch and you reach the wriggler stage (~60 hours at 80F), you can go right back to hard tap water.
Depending on how large the fish you're buying, you've got plenty of time. Get your fish started (change lots of water), then set up an R/O storage system somewhere accessible. I usually use a 29H or 37 cube to spawn, so you don't need that much storage. Good luck!