So I have an update to my post from a couple of weeks ago. I did a few full charging sessions and they all showed estimated capacity around 4700mAh:

Then yesterday I found this curious setting, after updating to Android 16 QPR1:

I turned it off, did a full charge session and I got this estimate:

And that result looks to be right back to the normal cycle degradation line:

I'll do a few more full charge sessions but we have a tentative culprit. Google seems to have introduced a battery capacity cap under this setting, presumably to extend the cycle life of the battery.