Yes, those WET MIX fades are more appropriate for XP-300 Space Station effects. Cool enough; just not ‘right’ for this application. You need a way to toggle these FX.
I’ll speculate that the Scholz mod used the footswitch in the commandeered Wah pedal to switch the effect – instant on & instant off. That option wouldn’t be available on most expression pedals. That’s where the suggestion for ACTIVE / BYPASS switching came from. Assuming that you wanted FX ‘trails’, I thought a BYPASS mode of DSP + FX was the best fit. Maybe DSP (only), or RELAY instead, for instant on/off?
You really only need to control DELAY times with the expression pedal for this effect; given on/off switching elsewhere. Perhaps a switch sending MIDI CC from your looper / MIDI controller? The only reason that I included FEEDBACK level adjustments with the expression pedal was to have some method of toning down & ‘erasing’ the effect between uses. You have to have a way to ‘feed’ fresh input into the delay oscillation.
Any FEEDBACK values between 100 & 110 (and sometimes as low as 85-90) will give you the infinite ‘sustain’. And that goes for any regenerated delay-type algorithm. I’ll toy around with a few things when I get back in front of it, & see what I can come up with.