Q: "how does the sound change when you alter the setting on the trim pot?"
A: Not much, except at the very end of the trimmer rotation, where it seems to act as expected, but not very convincingly. That's my concern, since I think there may be a better operating point if the control range was extended. I'll experiment with a lower value for R23.
Q: "with your current setting, what voltage are you getting on collector for Q6?"
A: Without signal I get about 4.7V. With signal, it goes down to a voltage between 0.7 and 2.7V, depending on the trimmer's setting, which is expected.
Q: " Usually a trim pot for a transistor will set the voltage at about half of the supply voltage"
A: That is true for a stage that operates in its linear region, but here Q5 & 6 are used as a switch so their state varies between open and closed.
Eventually I got somewhat more elaborated answers from forums, but basically it ends up being "tweak until you're satisfied with the result".
I don't think that's how it was done at Boss when they made the Slow Gear. It may be a suitable procedure for a boutique manufacturer, but not for an industrial product.