You cannot add anything to the water that's going to drop your hardness content, you need to remove it. It's got to be RO. Peat moss might take a tiny bit of hardness and lower your pH a bit, but with your hardness levels it would take two truckloads of peat pellets to move the needle at all most likely.
I use RO for fish I'm trying to breed that need softer water for egg laying. And it works OK, but even then it's a big pain and takes a long time to make water. And there's a ton of reject water, which may or may not be a problem for you depending on water costs. If your pH of 8.7 is real, it's quite a bit higher than mine is at 8.2-8.3. And while my discus have been "OK". I'm about to lose the 4th one of the 10 I started with. While none of them have been from "hard water". I think having them in water that's so conducive for bacterial growth probably helps contribute to issues longer term. That said, if I had to use RO to keep discus there's no way I would do it. Too much circus/work.