Your Oppo clearly has a better DAC than any TV. For that reason I would use either digital or hdmi from the TV to the Oppo. The Oppo is already connected to your pre, so you are good to go. The tv should output all smart sources to the Oppo, including cable. Just check "external speakers" on your tv set up.
Want it Incorporate video into current audio only system,
I have a rather large room which I use for watching TV and listening to music. I would like to be able to connect my smart TV to a preamp/ AV controller that could support a CD/DVD player, turntable, etc. I am not that interested in multi-channel operation but if that was the option I would not it rule it out. My primary concern is the best sound when listening to music. Current analog is Bryston pre, Levinson power, Revel speakers, Thorens turntable. Digital is Oppo 205. Considering Mac MX121/122. Is that overkill?