If you are planning on a Dutch type tank, and going to the trouble to make RO water, then I would just go 100% RO.
Then you can control every single parameter.
And I would not worry about too much about the low pH. Myself and many others use RO with very little to no KH. Unless your fish stock is a known hard water species, most do just fine.
Here's a good article you might enjoy.
https://www.2hraquarist.com/blogs/ph-kh-gh-tds/is-low-ph-in-tanks-due-to-aquasoils-softwater-a-concern?fbclid=IwAR0rqZ5z-6DSDYqeWRT-QB_2U3qFDt79A0U4oc7o21mUYjYIniT27bZ5CQg
Then you can control every single parameter.
And I would not worry about too much about the low pH. Myself and many others use RO with very little to no KH. Unless your fish stock is a known hard water species, most do just fine.
Here's a good article you might enjoy.
https://www.2hraquarist.com/blogs/ph-kh-gh-tds/is-low-ph-in-tanks-due-to-aquasoils-softwater-a-concern?fbclid=IwAR0rqZ5z-6DSDYqeWRT-QB_2U3qFDt79A0U4oc7o21mUYjYIniT27bZ5CQg