eARC is available on HDMI 2.0 as well. The Marantz SR6013 supports eARC and of HDMI 2.0.
I faced somewhat of a similar issue with my G1 where initially, it was only sending 2 channel sound to the AVR from my PlayStation 5. I thought it was a cable issue so i ordered a certified HDMI 2.1. while that didn't solve the issue, it made the audio sound worse in two channel mode (which was strange since HDMI is HDMI right? Plus, it was an expensive cable.) Threw that into the bin and reverted to my el cheapo amazonbasics hdmi 2.0 cable and thereafter, enabling eARC and Digital Sound Output as Pass Through in the sound menu finally solved the issue for me.
Now, the chain is PlayStation 5>>HDMI 2.1 cable>>LG G1>>HDMI 2.0 cable>>Marantz SR6013.
I tried old HDMI cables as well and still it is not working. I have given up now and have connected my Nvidia Shield directly to AVR . Don't need passthrough anymore