I've been following this thread with interest as I have an RX-V6A on order (about 2 weeks away from delivery), to replace an older Yamaha RX-V673 which is only 4K@30fps.
I only recently upgraded from an old LCD to an LG CX and got ARC working in my system. Despite being a Harmony Elite user, I leave the CEC and ARC enabled, as it's useful. But ultimately, ARC can always be replaced by an Optical cable which will work just as well but without CEC features.
So I don't quite understand why people are plugging all their devices into the TV and trying to use a single cable via eARC (which is a HDMI 2.1 level feature by the way) to connect to the amp. Eventually this will be possible but right now, that firmware is not there yet.
Also, can anybody answer this? Is 4K@60 and 4K@24 passthrough working OK through the amp?