DOH! That's what I get for researching in the middle of the night.Dante from the SDP-55 is for output from the SDP-55 to power amplifiers only so wont help you (Dante doesn't really work in the current firmware either). Video card is definitely the way to go for HDMI audio output and using the spare HDMI output on your 3090 would be my first choice.
During HT I will have a spare, but not during gaming.
So, I will use the 3090 for video output and get some other HDMI video card for audio only and try that.
However, thunderbolt 3 does 40Gbps. There's not a way for me to use those ports for audio out by getting an adapter?
(Oops, I never mentioned I had 2 thunderbolt 3 ports. I have an Asus Maximus xii Extreme motherboard, if that helps).