If your pH gets lower as the grow progresses, it's probably from bacterial acidification. Since I sterilize my peat, there is no acid producing bacteria. I add seven strains of myco after sterilizing. They don't produce much or any acid. My tap water has 60ppm bicarbonate that will gradually raise pH. Maybe you are using RO water with zero bicarbonate?
I don't have a reliable formula to share for adding potassium silicate to raise media pH because I don't have to do it often. 500ppm of potassium silicate soaked into the plantless media for 24 hours sounds like something I did once with success.
It sounds like incorporating a small amount of potassium silicate in your normal feed might help stabilize your falling pH. That is one reason why I don't use potassium silicate in the feed and use liquid bioavailable Silica instead.