dabarrie OP
I am confused by your question.
Are you asking if you should use your existing 2 channel receiver in conjunction with a separate surround processor or just a a/v surround receiver for all channels instead?
If so, I would say go with the surround receiver for everything. Everything will work in concert with each other (Atmos, crossovers, levels adjustment etc.). If streaming audio get an independent DAC and connect that between the source and the receiver for better audio quality.
Both the Onkyo and Yamaha look good, although I have not heard them. I guess it would be a matter of personal choice. Take a look at Marantz also! For what it's worth, I had a Marantz NR1510, their cheapest model with 2 subwoofers and although only 50 watts per channel was more than enough to fully enjoy anything I ever watched with it. It only lacked Atmos, but I never missed it. I upgraded since, but not because it sounded bad but because that's just my nature. I wanted separate amplifiers for every single channel when I built my video room about a year ago.