You are in soil. Unless your water is really bad, there isn't a need for RO. And to flush soil you need to add a min of 3 times the water to the pot size. So that being said useing 9 gallons of RO water waste 18to27 gallons of water. Good for feeding, you know what's in it. But I think it's a waste for flush. Soil will leave trace elements anyways so I would say just use tap as is. Or if your bent on RO. Flush means adding nothing. I'm hydro but when I flush I don't even ph the water. Don't mean to be mean or start anything just saying how I see it.
Oh I just had an idea. If you want it as clean as possible. Why not flush with tap first then flush with RO? Good looking plants man