What I would do is add Ca Carbonate until I met target GH target, then add additional baking soda till I met KH target. KH will drop over time at a rate dependent on the amount of acid being produced, i.e. nitrate in water is nitric acid and phosphate in water is phosphoric acid. KH drops as carbonate plus 1 Hydrogen ion from an acid makes bicarb and bicarb plus 1 H ion makes water and CO2 which exits the system. The elimination of the H+ is the buffering effect.
If you find yourself having to continue to add bicarb to maintain KH you still have a source of excessive acid production, if you are not sure you can track with a TDS meter, which does not actually measure TDS but all ionically active dissolved molecules and then estimates TDS. The obvious next question is how much is too much. Given how soft your water is I assume you are starting low, and stability is more important than absolute number. I would check your TDS AFTER your have fixed GH and KH and then if you are having to add additional NA bicarb (your GH should not go down) then check your TDS and probably opt for new water over more bicarb at 2x starting TDS value, but I have no good data to support that opinion. Curious about other folks thoughts. Hopefully this last is pure speculation and fixing KH fixes problem.