OK, here's an interim update on how things are going (ONLY one week in) with my new replacement SP4 hardware (Lot 1618, so probably around early May, depending on the week calendar Microsoft uses: ISO week 18 was May 2-8 this year).
The good news is that for the past week, I have only suffered one conventional Sleep O' Death or Dark Wake failure, and that one was uncharacteristically immediately after UNdocking. I have never gone this long without a SoD failure, with either of other
two SP4s. I'm beginning to lose a bit of my terror to put the machine to sleep or wake it. I will say that while this one wakes, it's sometimes not instant, and can take 5 seconds or so, plus another 10-20 seconds before Windows Hello wakes up, recognizes
you, and puts you back into your session. This is noticeably slower than my previous SP4, discounting the reboots - weird.
I also suffered an unexplained mystery crash in the middle of the night last night, while the machine was (or should have been) asleep. Here's what the Reliability Monitor has to say about it - of course there are no solutions for the hardware errors, which
are for PCIE and USB3 hub hardware...

It's a bit too early to say for sure that the newer hardware truly avoids SoD problems, but it's starting to look that way.
I'm reserving final judgment until after the August 2nd Redstone update, since we've seen a couple of previous updates make power management and sleep/wake/dock problems much worse.
So far, so good though - it looks like there really may be enough of a difference in hardware to actually make a difference. Another week like this, and I'll likely have more successful wakes on this SP4 than seven and a half months of using the other two
SP4's combined.
I'll keep y'all posted when there's more to say...