I did another factory reset and this time did no restore. I actually disabled the backup during Setup. So, pretty vanilla. And...
No better. In fact, while it is plugged in, the phone went from fully charged when I started the reset, to 86% when I took a screenshot at 2:25 PM, to 47% at 5:06 PM (yes, still plugged in).
Also, I have been getting the message (before the reset and after) that "Google keeps stopping" and I am now at 44% at 5:13 pm.
And going into the Settings\Battery, it shows "Phone used more than usual" and opening that, the same Mystery App is the only "Top apps by battery usage" listed, as shown in prior screenshots.
FWIW, I replaced the battery when the battery wasn't holding a charge all day and, some days, I had to recharge 2x. But this drain is rediculous, as I'm not using it at all, it's draining when on & the screen is dark and it's "charging (plugged in)."
Is it worth trying a different battery? This means opening it up again, of course.