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

Hi svbriggs, welcome to the forum!
RO water was stripped out all minerals. The following product is just what you need.
Seachem Equilibrium RO Water Conditioner
It can rebuff and re-mineralization RO water.

PH in the range of 5 to 7 is perfectly fine. Lower than 4 (might) stall fishless cycling.

I am glad you are doing fishless cycling with the right products (One and Only, ammonia).
On a side note, you might want to keep up with the partial water changes in your betta's temporarily housing since it is not the ideal size for long term staying.

Good luck!