Nice AV receiver you've got there

. What's your PC specs?
Personally, I'd aim to use HDMI audio passthrough rather than going down the HDMI video / Optical Audio route. If you've got a modern PC, you may find you can already get great audio passthough from your system over the existing HDMI.
My old HTPC system could output great 5.1 audio over HDMI to our Sony 1040 AV receiver (not Atmos like yours) and the quality was fantastic, better than optical out from other solutions.