Hello M00n,
1. I would think that 2 WC per week (being at minimum a 50% change) with that tank size, filtration, and stock level would be sufficient. More WC is always better though.
2. From what I’ve gathered from my use of RO water mixtures is consistency being the most important. You don’t want wild pH swings from inconsistent mixing. When I did RO I was doing a 50/50 mix (7-7.5pH) and my discus seemed to do well. I live in Texas and have pretty hard water. Domesticated discus can thrive in various pH water though. I’ve heard people raising them in levels as high as 8.5. Consistency again is the key here.
3. When it comes to water quality discus demand the cleanest if you want them to be large and healthy. What I’ve learned is Discus are pretty susceptible to illnesses if kept in subpar water quality no matter where they come from. All I know is all discus came from some of the cleanest water on earth so in order for them to reach their potential you have to mirror those conditions.
Hope this helps