You don’t need to create an aggregate device to work with Logic, although I guess that way might a bit simpler as you’d have access to the Overbridge channels in exactly the same way as those from your soundcard. The way I do it is to add a new track in Logic with Overbridge plugin; select the configuration you want to add while adding (for the Digitakt, I use Multi-output, 1 x stereo and 9 x mono). This brings in the main audio output. Then open the mixer (press X).
See that little + icon over the solo button? Keep clicking that until all of the Digitakt channels appear as Aux channels the mixer.
You can then send the output of these aux tracks to individual busses, and then set up audio tracks to capture the audio on each bus coming out of each track, apply FX, EQ etc (this may be an overly complex way of doing it, but it work for me)
To capture the FX track in stereo, you can change the voice routing so that only the FX track is going to the main outs.