Well, let me put it this way:
If I had 16 channels of DA analog conversion, I'd be looking for an analog mixer.
If I had a soundcard with 16-24 channels of digital connections, I'd be looking for a digital mixer.
The point being, I would not want to subject my music to all those conversions if you are going from your DAW to analog, to digital again into the mixer, then to analog again if you are going to a compressor or eq, then back to digital to get it back into the mixer. I'm assuming that you can take the stereo mix from the digital mixer digitally into the DAW.
So you need to look at the big picture and view your mixing setup as one system. Make it work how you want it to work. If you want to mix on a digital mixer, then you should have soundcards that support that so you can avoid unnecessary conversions. If you want to continue to use your current soundcards, then at least think about how best to mix.
In your situation I might not even get a mixer, but connect my outboard to the soundcards, and use them basically as busses and fx sends from the DAW mixer. There are so many ways to do this it is mind boggling.