Hi, here's an update. Three weeks ago I uninstalled a couple dozen apps, and inactivated another couple dozen apps. All I have left are three drawing apps, and a handful of others (Uber, ASTRO, etc.). My battery life is really good now (although that's probably just due to the new battery). Most important of all, performance is really good now (no more sluggish behavior). Initially after doing this, the "Android is upgrading" message continued to appear whenever I did a reboot. But it only lasted a couple minutes so I figured that was good enough.
But then, in the past couple days I suddenly noticed that the "Android is upgrading" message no longer occurs! And reboot time is shockingly fast (by my standards, for this device), like I don't know, maybe 5 seconds or something. So that is really nice.
It is mysterious how the problem took care of itself, or at least not as an immediate consequence of the actions that I took. I am certain of this because I kept track of these changes in writing. I only use this device every few days so it's hard to know when the change occurred.
