I find FaceTime works much better on 4g than my home wifi despite good upload bandwidth. I suspect variable latency is the cause. My router and Orbi does not have Ipv6 enabled. If I can get V6 working will I see significantly better video quality?
It could be a case of the two parts of the conversation not being able to make a direct connection to each other because of NAT, CGN or similar, and therefore falling back to an intermediary proxy server. That will introduce latency, and could potentially cause lower perceived quality.
Enabling IPv6 could solve that - if it’s not encumbered by the same problems by your ISP or equipment. IPv6 is usually much better in this respect though.
You could also try to make sure that hole punching is possible on your IPv4 setup.