HOLD UP!
If you’re able to work with audio over USB for the FX loop, the Audio Routing menu in Settings has options for USB OUT: you can choose individual tracks for L and R (or Main or Ext), and then USB IN can be Pre-FX or Post-FX
Sadly this isn’t going to help with the Strymon reverb or other guitar pedals, but I think—will test this out at some point—that routing option opens the possibility of passing a couple of tracks through effects on an iPad, which could open some interesting possibilities…
This is an option on Syntakt that’s not there on my Digitakt, but the answer above about Main, Headphone, and USB being identical for ST was wrong…