Someone please correct me, but I believe peat under the substrate doesn't enter the water column and effectively lower KH, but rather breaks down and feeds bacteria (which can be added as mulm) in the substrate to help them break down nutrients and feed the plants. It doesn't need to be replaced with time and rather jumpstarts the substrate. For example, ADA Aquasoil contains peat and its effect lowering KH runs out after a couple months, while remaining an effective substrate. (Many replace Aquasoil with new Aquasoil to kee its magical goodness, however). I have an ex-"natural" tank with peat under Schultz (inert substrate) and its still going well a year later, fwiw, and I have no plans for replacing the peat.
The most effective way to soften and maintain target KH is to mix tap with RO/DI imo. I keep a softwater tank and simply increased the RO/DI percentage as (~7month old) Aquasoil lost its effectiveness. It is not as difficult as it sounds, and RO/DI is only 20cents/gallon around here. Bonus is I know I keep some Ca/Mg/tap traces as well.