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

Equilibrium is for adding buffer into RO water for more stable PH.
It does so by restore the minerals. RO water has 0 mineral and thus 0 buffer. So basically adding mineral into water is the same as adding "PH buffer". The amount of dissolved mineral in the water will decide whether the water is hard or soft. Harder water has higher buffer, and thus harder for PH to fluctuate.

What you shouldn't do is to use those "PH up" or "PH down" products.