More less the same with me, I'm a console gamer but I've been slowly switching to playing games on PC due to convince (plus the damn PS5 is still hard to find). With the existence of the Steam Deck, I can easily take my games with me and continue on the big screen when I get home which feels like magic.
To get your AV receiver working with your current output, you could use a HDMI Splitter, but at least in my experience, it tends to strip the digital audio. Most modern motherboards can output surround sound no issues and the HDMI from your videocard should be able to output audio no problem. The thing about an Optical or coax cable is that it's limited to DTS and DD5.1 so it's kinda outdated and blu ray movies (and games) have taken advantage the newer codecs. Of course at the end of the day I'm just generalizing here since every hardware is wildly different, your best bet is to probably start with a "premium certified" HDMI cable from your PC to your AV receiver and see how it works out from there.