After a few days of trial and error I luckily found the solution, but nothing like this is explained well enough in the manual, that could be way better!
One has 8 DAW direct outs (why not 16 ??? that would have been much easier if you want to fully use the 16 outputs all together).
F.e. I also teste a Focusrite 18i20 and that one in the software mixer you can route every input and every ouput to your wishes and that is how it should be.
My first goal is to not use the UR816C for recording but for using the ins and outs for external hardware into Cubase like plugin-inserts.
In Cubase you get in the input list all the 16 inputs(ADAT included), which is offcourse the easy and right way.
But outputwise there are only 8 DAW outputs, the rest are 4 mixes L+R, which is not that obvious.
That I can also use it for recording is a second benefit for me, but not that important. For recording offcourse those 4 mixes-possibilty can be usefull.
Now I found that next to the 8 DAW Direct outs I have to use the 3 mixes(MIX 1/2 is and stays for Mains out, you have to keep on using that one)
for my further 6 outputs (instead of 8).
I now use the MIX L+R of these 3 mixes to feed external hardware. It works but you have to be carefull and pay attention how to set up these mixes, every input channel has to be muted for not getting feedback. It is not that obvious I would say but it works for me and I am glad that I can keep using his interface.
It works just fine now with the Behringer ADA8200.