Regarding acid additions, Phosphoric Acid is more flavor neutral than Lactic Acid. Now since you're using RO water, the quantities of acid used will be small, with no apparent flavor contribution when using Lactic acid.
Pricewise, however, typical homebrew store and online sourced Lactic Acid comes as 88% in 5 oz (~150ml) bottles, while Phosphoric Acid comes in 8 oz (~240ml) bottles at
only 10% for roughly the same outlay. Thus, Lactic Acid is by far the more economical choice. For ease of measuring those small quantities, best to pre-dilute that 88% Lactic Acid to 5 or 10% or so, and store in a well marked plastic bottle, in a safe place.
If you have need for larger amounts of acid, you can buy 85% Phosphoric Acid (e.g., DudaDiesel) in quarts and gallons.
I split part of a gallon of 85% Phosphoric Acid with other homebrewers at our group grain buy, and still have enough left for 10 homebrew lifetimes.