Discovered the 7 taps to enable Dev Options shortly after posting the above.
It didn't change anything.
So, currently:
1. DevOptions->Root Access shows "apps and ADB"
2. I've rebooted into TWRP and have tried (either both or singular) "Advanced->Fix Permissions" and re-installing "TeamEpic-Root-from-Recovery-v5"
If I re-install "teamepic-root-from-recovery", on boot into the phone, I'll see a google message about "updating apps" before getting to my lock screen.
I re-enter Settings and verify "Apps and ADB" is still set, then launch SuperSU and the "Update->Normal" path contiues to result in "Installation Failed!" message.