RO water out of the tap, what are best products for conditioning and restoring kh?

Thanks!!! Forgive me if I seem dense, I'm just trying to make sense of it all and not do anything to injure my fish.
I will on your advice ditch the pH buffer and stick with Equilibrium/RO Rite.

I already added the pH buffer to my cycling tank (it only raised pH to 6.8), I'm assuming that since it is phosphate based, I will gradually remove it with water changes, correct?

So back to my water changes for my betta in his holding tank. My tap water + conditioner (prime) + Equilibrium/RO Rite has a pH of 6.4. My holding tank has pH of 7.4 ish. I found that 10% changes over a half hour doesn't stress him and doesn't swing the pH. What ever is buffering the water in his holding tank (substrate?) Is maintaining stable pH. Do you recommend that I not mess with trying to raise the pH of my tap water (the water that originally went into his tank) and just continue with slow water changes of 10%?
Also, should I remove the carbon filter from my cycling tank? Will it mess up the cycling process? It is just Day 1 of cycling.