I think you are imagining the buffering ability of peat to be more than it is and the permanence of phosphoric acid to be more than it is.

In year 1 I replaced 90% of the ground soil with peat moss for a blueberry planting. Each year I added some elemental sulfur. In year 4 the planting was destroyed by a couple puppies I brought home.

Before purchasing new shrubs I tested the soil. It's pH was 7.2. The surrounding native soil is 6.8. This was in a large area, much larger than any container and it was the highly alkaline irrigation water that raised the ph of 90% peat above that of the surrounding soil.

Sulfur added annually breaks down to sulfuric acid, pretty strong stuff, yet not strong enough to counter the constant liming effect of the irrigation water.

Vinegar, added to the irrigation water, is very effective. It stops the problem before it can start.

Commercial pH down products are simply a waste of money. They are simply acids. Just like vinegar. The only real difference is they cost more.

As an addendum, I measured the pH of the leachate coming out of my similar to 511 mixes after 1 year of use. It was also 7.2, the same as my highly alkaline water.

Bottom line: If your irrigation water is highly buffered at a pH above what your plants can live long and prosper in, address the problem at it's source. Add vinegar to the water or use rain water. If the irrigation water is chemically out of whack, there is pretty much nothing you can do to the mix itself to counter it. Every time you water the out of whack chemistry does it's nastiness. Correct it at it's source and don't worry about the media itself.

The media is just there to support the plant without killing it. Everything else is up to you.

Commerical pH Down is primarily used by hydroponics. If you search on the internet, weed growers have a lot to say about pH down and vinegar, in hydroponics or in soil/less medium. Some people had tests that adding vinegar into the soil medium, in an hour, the leache pH is about the same as original container mix. While pH down last a lot longer. It has been almost 20 years since I learned the Chemistry. But I still know the basics very well. Still I can't understand the difference.

I water pretty much using rain water and snow water all the time, rarely with tap water and my tap water is soft, about 40. My peat based mix is having a pH of 6.5 after a year

Some people had tests that adding vinegar into the soil medium, in an hour, the leache pH is about the same as original container mix. While pH down last a lot longer.

They are both acids. One is weaker than the other.

Trying to control pH inside the container is largely futile.

If one's water supply is chemically out of whack (high alkalinity) then this can be corrected with any acid in an appropriate dose in the irrigation water before being introduced to the container.

Nothing more is necessary. If the media's contribution to solution pH is acceptable then the only other factor worth considering is the irrigation water's influence on pH.

Vinegar can deal with any alkalinity issues in the irrigation water as can any other acid.

If one chooses to spend $10 on a 10 ounce bottle of pH down it will work fine. If one prefers to spend a couple dollars on a gallon of vinegar, that works fine too. It's just an acid intended to produce a desired chemical reaction to reduce/eliminate alkalinity.