Oh, I didn’t see that you knew that it was a phase issue, but that post should help someone. If there’s a reported 2ms signal delay like you said, there’s no way around a phase issue using aux without somehow delaying your dry track by 2ms. Using aux for delay is usually done post fader with the full wet delay line coming in from return and the mixer channel the dry signal, since it’s delayed anyway phasing usually won’t be an issue. Or you can go pre fader if it has a wet/dry knob.
Edit: I should have read more of this thread, I assumed deco was a delay. Seems you want to not use a parallel aux send with that unit. Sometimes when I’m tired I respond without knowing all the relavant info, got to work on that…