Hi Rafal, I don't know how much processing you do, but it might be worth considering the option of replenishing your fixer.
This is the standard method used in high-volume processing. Whatever is wrong with the tank solution is fixed with the right amount of the right replenisher. (Replenisher volume is normally based on area of the material being processed.)
Since you indicated that you could mix to a certain pH aim, and you know how to limit your first fixer to a certain silver-content, I'm thinking that you could probably customize your own fixer replenisher. (I wouldn't normally suggest this idea, but it sounds like you're having fun formulating.) Essentially you would make a fix replenisher that counteracts the pH-lowering effect of the stop bath.
If you're interested, and want some suggestions on how, I'll be glad to kibitz.
ps. of course, Gerald's suggestion is much simpler and gets right to the point.