Interesting ... my purchased in October 2020 X4700H developed these EXACT same problems (and tried the same troubleshooting steps to no avail) ... only mine was just days OUT of warranty

It did not however quite degrade to the point your did ... sometimes it would still work okay for hours. Other times it'd glitch every two minutes.
Took me a while on the test bench to figure out solid 'steps to reproduce' so I could pass them along to the repair center. I chose United Radio in NY even though they're like the furthest from me, based on quality CS of the person on the phone, a flat $200 to fix (+ parts), and the fact they charge nothing for diagnosis.
It's currently en route. When you received your X4700H back, had they repaired it or just replaced it? And if the former, did they, by chance, inform you as to what exactly they DID to fix it? I'd love to pass that along to United cause, well ... sooner I get it back, and working ... the better.
I discovered that the glitch is most likely to occur quickly if you try to operate the unit immediately after plugging it in.
Also very disappointed in Denon on this one, I have LOTS of pieces of Denon gear from the 90's and 2000's and they all still work flawlessly since the day I bought them.