What do you hope to achieve with the mixer bit ?:
1. Analogue sound? Routed as you suggest you get analogue front end sound but you could do this with a channel strip with built in ADC as opposed to a full on mixer. If it's analogue summing - then you need an analogue hybrid mixer with 8 digital inputs to connect to your interface .
2. Connectivity of external processors and monitoring? Only on 2 channels of audio because you're only coming out of the DACs in stereo - that might be the aim, if so, what is that for mix inserts or fx sends or purely monitoring? If more channel processing required simultaneously you again need a mixer with 8 digital inputs as well to fit your converters.
3. Hands On Tactile Approach - a bit improved on the output stage but again - only on 2 channels unless you go for a more involved hybrid desk with 8 digital inputs/ eq/ inserts/ groups/auxes as well.