The manual says that the ARC functions the same way as your Beam i.e. the sound from HDMI sources to your TV gets passed to the TX-L20D. There are a couple of things to check. First, your TV is ARC compatible; which given what you said about the Beam should certainly be the case. Second, that HDMI CEC (HDMI control) is switched on in both the TV and the Onkyo. The default value is OFF with the Onkyo out of the box.
HDMI CEC should also fix the lip sync issue with the Panny. Make sure CEC / VieraLink is enabled in the Panny too.
Did you check - or did someone who made the recommendation say - that the set had Dolby Vision?
Nobody recommended the onkyo I picked it myself. But I primarily hoping to have the same easy setup as discussed via hdmi arc as Sonos. I would only use the Xbox one x in the onkyo at push as all my other devices do DV .
I’ve just tried doing optical from tv to the optical on the onkyo and using The panny as normal ....hdmi from player to tv.. The sound issue is fixed... although this is a slight faff as I’m now using yet another remote for sound.
i’ll retry The hdmi arc with settings you suggested Lucid... Although how do I change my other devices ? Apple TV being one of them.