Not sure if this will work, but you might try an Oppo 103/105/203/205. These have HDMI inputs you can use and can be configured to down-mix everything to 2-channel PCM on the S/PDIF output. It might work for you.
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Thanks for the suggestion. I'm giving this a shot, but unfortunately you don't have any control over the downmix. https://www.parts-express.com/parts-express-hdmi-audio-extractor-with-toslink-and-lr-audio--180-995
I'll keep the oppo option in mind...