In the Franck Chorales, you can hand register most of the incremental changes when one hand is free, or use a few divisional (scoped) pistons where that's too awkward, for example if it means poking at a touch screen.
The big changes do work by presetting the Anches (upperwork and chorus reeds) by hand and using the ventils to bring those stops in and out, and I find it necessary to do that with my feet most of the time. I have a partly disabled left hand, so pressing thumb pistons while both hands are playing is difficult.
I'd say the minimum toe studs are 4 ventils, 3 manual couplers, and at least the G.O to Pedal coupler for Franck. The GO to Pedal transmits through if other manuals are coupled to the GO. But for Widor you need all 3 pedal couplers as toe studs. So 8 or 10 for basic functionality. The Caen sample set has 15 foot switches.
If you look at the available pedals on St Clotilde in Franck's time, there were also suboctave couplers for each manual, but no Recit to G.O. coupler. He had to couple R to Pos and Pos to GO, which transmitted R through. Same for R to Pedal.
After playing with the ventils for a while, I ended up using a uniform piston layout for all the Romantic/Symphonic sample sets that works on English and Americsn organs, too. I kept forgetting to hand set the reeds and upperwork on the CC and Mutin organs, then hitting a ventil and cursing myself when nothing happened. So now toe studs add the stops (reversibles) and the ventils are always on. Lazy but less embarrassing for me...