@Ralf The polymerization of the silicic acid is quite fast at pH 2-8 . If you want to lower the pH, then you need to do so as fast as possible to the <2 range. The key here is that this pH reduction needs to be as fast as possible (acid addition needs to be concentrated) and the silicon needs to be at the lowest concentration possible.
About the stabilizing agent addition, I really haven’t tested the effect of adding the stabilizing agents before or after the acidification of the silicon solution. It might work, depending on the stabilizing agent. However doing the addition at basic pH really helps solubilize solid potassium silicate sources (like AgSil 16H), when you’re starting from them.
Let me know how your experiments go!