This is actually quite easy, because you can use Stagebox Mode and do all mixing on the console. When both mixers are connected via AVB, go to "Audio Routing" on the touchscreen of your console and then "Stagebox Setup". The rack mixer should be listed there already. For Preamp Permission select "StudioLive 16" and as Mode select "Stagebox". "Apply" the setting - and that's it...! Now the 16R is working as stagebox and all mixes are made on the SL 16. A nice feature is that you can use the 2+6 outputs of your mixer simultaneously.
It may be helpful to reset project/scene and networking routing of the rack mixer first, because Stagebox Mode is a network preset, which works best when mixers are "fresh". I always recommend to read the "StudioLive_Series_III_Stagebox_Mode_Addendum_V5_EN_25032019", which you find in the download area of your mixers.
On the console you can toggle between DAW and normal mode. You don't need to do the monitor mixes on the rack mixer.
