Sphagnum peat moss alone has a pH around 4.0, so I don’t think I’d really call it a buffer. It’s most often used in gardening to decrease soil pH for acidic soil loving plants (blueberry plants, for example).
Also, depending on your methods you may not have or want runoff. No-till containers, for example, should not have much, if any, runoff. The idea is to keep your soil consistently moist, so you water smaller quantities more frequently.
I’m right there with ya. My tap water comes out around 7.9-8.1 pH and isn’t particularly hard, but it’s not soft water. I’ve tried a couple times to avoid adjusting water pH in small containers (3-5 gal) and larger 20 gal containers. In each case, the plants quickly remind me that I need to give the water pH some attention–especially in new containers. I don’t pH to an exact number each watering…I shoot for water that’s between 6.0-7.0 with an overall average around 6.5.