An important understanding is that you’re still going to make beer in almost every case. Starting with RO water makes that even more likely.
I council brewers to avoid thinking about brewing water needing to be exactly so, for a particular brew. I prefer to use the concept of getting “in the ballpark” for brewing where there’s a modest mineral content and the water has a hope of getting the wort pH into an acceptable pH range.
Using a brewing water program helps get your water into range, but just using the simple water adjustments recommended in the Water Primer thread on Homebrew Talk will help get things close. A brewer can then refine the next brew with a bit more or less of this or that in the brewing water.
If starting with RO and malt extract, I’d say that you’re on a path to success with just that. But to help bring out more IPA character, adding gypsum to the water will help. I don’t recall what the recommended amount was in the Primer, but I’d be targeting at least 150 ppm sulfate.
You’re well on your way with your current plan. RDWHAHB.