Good news (sort of) is after 3 years the Simplink now works OK most of the time. Whatever was causing the TV to not turn off when powering down Apple TV or the DVD player has disappeared, most of the time. The Apple TV OS has been updated a few times since 2017, that may have helped but I can't be sure.
My Verizon set-top box is still the PITA because it doesn't support HDMI-CEC so I have always change the input manually when watching cable after watching a DVD or Apple TV. I still have FOUR remotes to deal with: Verizon remote, Apple remote, TV remote, DVD remote. The only thing I use the TV remote for is changing the input!!! Whoever thinks this makes watching TV simpler needs their head examined. 😡