Had to check in here tonight to provide another update--as it wouldn't be cricket if I didn't, imo! I just spent the most pleasant day & evening in many weeks watching several disks, DVD, 4k & Blu-Ray, (all scaled to 2160P) with my new Panasonic UB-420-k! I had actually forgotten what it was like to watch several hours of disk-based movies (and stream--but a ROKU Ultra does my streaming now) without a single, solitary freeze-frame/pwr off and/or Factory reset! My wife was ecstatic. Nirvana has been achieved..
Operating on the same exact network as the x700, reliability and therefore performance is demonstrably above the x700--I was surprised, really, at the differences that were immediately noticeable on the first day.
Even the bios option UI presentation and logic is head & shoulders superior to the x700, imo--now, that's the way a bios UI should be presented! Comparatively, the x700 looks like a really almost sloppy cluster of software add-ons, extensions, and literal afterthoughts. I mean--to my eyes it is exactly as if Sony was shoehorning options willy nilly to meet bullet lists of "features" with very little pre-production planning (such as the late software support of DolbyVision, for instance, which came through a software patch after the x700 shipped.) The 420 looks and presents as if designed from a carefully thought-out master plan--the x700 simply doesn't present that way at all. (Dolby Vision isn't a concern for me--because the 420 supports HDR 10 & 10+.)
That such is the case for the x700 is fairly obvious by way of Sony's official tech support advice for power cycling and/or resetting all the options to factory to try and cure the horrific freeze-ups and lockups. I didn't see any similar "support" messages from Panasonic concerning its 4k disk players, nor was I successful in finding a copious amount of "my 4k player is freezing up constantly" for the Panasonic offerings as I found easily for the x700--page after page of complaints--and after today's freezeless marathon performance of the 420, I can see why! When you don't get freezeups, you don't write complaints about freezing up, etc...
These are real, serious problems for which Sony has done nothing
except to offer the very lame and sufficient "power cycling and reset to factory defaults" advice & followed by "you may have to send to service if these things don't work"--that advice never worked for me permanently despite many weeks of trying everything I could think of along with following a lot of well-intentioned advice (here and elsewhere) for which I am grateful. But it wasn't enough. The x700 screams for a thorough bios update, for instance, but I could find no indication that Sony has any further plans for the x700 in that direction.
--Another thing I noticed with the x700 is that undoubtedly heat was on the minds of the design engineers as they moved the power brick out of the x700 case to the power-cord plug. The Panasonic retains the power circuitry inside the case instead of external to it because overheating of the 820/420 was very well considered and then eliminated
in the basic player design. Size of the x700 case & the UB420-k is very similar, too
__Last thing I will comment on, is I much prefer the Panasonic remote! It isn't backlit, either, but it is a good bit larger and that's great protection against hitting the wrong key accidentally, I've found. Aside from being easier and less club-fingered to handle than the Sony remote, I find the key layout far more intuitive than the Sony keys--but that impression might simply be because of the comparatively cramped nature of the x700 remote.
In closing, I have nothing against Sony--of course not. Indeed, I bought the x700 after having been so impressed with my last Sony BR player & streamer--which to my knowledge never locked up once in years of use! But something happened with the x700 on the way to market, and thus my problems have existed as I have recounted them here. As mentioned earlier, I might be more skeptical as to the origin of my x700 if it looked like the problems I've reported were rare among the x700 user base--but when I can find many Internet forum pages in which people report the same problems I've had, I really have to dismiss the idea that I purchased a cheap knock-off product direct from Amazon
(I didn't buy from a third-party AMZN retailer, etc.) No, I think the design as it sits is flawed in different ways--but not enough that 100% of production units are affected, of course...
Thanks again for the kind suggestions I received here in these forums--there wasn't a one of them that I ignored...
Now--it's back to enjoying my movies & music, etc.! Delighted to do so!