con is with, di apart, both in the "is apart" and "drifts apart" way, also "between" and "not", and trans is, well, also apart, but implying some sense of border, not just (conceptual) distance. I'd say that digress and transgress are comparatively synonym (if you squint in just the right way) and both antonym to congress.
intragress might be an alternative to the missing cisgress, especially as ingress already exists. And then we could have extragress for being not on the inside but not beyond the pale, either.