You have a liquid test kit right? Just test your pH, do 1 cup R/O water mixed into your tank and test it. If it barley shows or not at all, you can use more at one time. If it spikes too high use less at one time.
I pers never done the R/O -Tap mixture, I only used peat, which worked great for me, but I also didn't have quite the sky rocking pH like you have.
And neither one of these versions will cause algae issues. Chemicals might do this but not R/O or peat (which is why I pers believe to keep chem's outta your tank lol)
There's really NO telling how your water may become over time. I had 7 tanks set up at one time, all same source water and ALL had different pH & hardness, ALL of them, even the one's with the exact same gravel used....That's why the fish world is so complex and there's always at least a handful answers to one single question
You're on a very good track there, I think you'll do fine over time. Add some plants as/ when you can, let that run for a while to establish properly with you Axo in it and add fish few weeks down the Rd slowly (and that alone there will change your water readings AGAIN lol).
Would be neat to follow the process of your tank if you felt like keeping us posted! Also will help other newcomers later on to be able to learn from your progress steps