In a final attempt to debug the long audio delay issue, I disconnected HDMI and connected my Samsung TV's optical output to the A6A input. There is the same 15-second or so sound gap as over HDMI when starting to stream the next episode in a series or switching from a streaming app to an OTA channel on the TV, as well as a 2-second or so delay when switching between OTA channels.
Then I connected the same optical cable to the RX-V2095 receiver (over 24-years old!). No such delays whatsoever, and a clear indication on the front display when a Dolby Digital stream starts and stops. Clearly, "they do not make them like they used to"!
So the conclusion is that, after a two-week evaluation, the A6A has to go back to the store, which is very unfortunate given its size and weight. I strongly suspect that this audio delay and the HDMI 2.1 8k/4k@120 issues cannot be effectively resolved in firmware, so a board change will be required (as hinted to me by one Yamaha customer support rep).
But I will keep my fingers crossed and continue to monitor Yamaha's progress on the RX-A6A. Based on bench tests published on another website, Marantz SR7015 and Onkyo TX-RZ50 are not recommended, and, on a second try, Denon AVR-X4700H is but it has only one 8k-capable HDMI input.
