Since it's already completed, I'll suggest the easiest route (IMO) that doesn't require desoldering/resoldering components nor drilling a new hole in the enclosure, but it does mess with your lovely ribbon.
Desolder the cable between the 3PDT and Board's input, insert a small trimmer inline between the 3PDT and Board-input. Try a B50k or B100k (experiment, find what you like maybe only need B20k) — I'll recommend one of the Bourns multi-turns (8 or 20 or something) so it's not touchy and you can get it Goldilocks' "just right".
Footswitch > lug 3 and Lug 2 > Board, Lug 1 > GND.
Now you've got a pre-gain control (set & forget) and can crank up the master!