Thanks again, but I already tried removing sixad as mentioned in my original post.
I just tried it again, but it makes no difference in the behavior of Register and Connect, I just get the blinking cursor in the lower left and nothing happens. I waited a good five minutes just to see if anything would happen.
To describe what I did, I ran sudo apt-get --purge remove sixad
Then I ran set-up, Updated scripts, rebooted, and went back Register and Connect Bluetooth Device, and it’s still not doing anything.
Also tested “bluetoothctl” again after removing sixad, and it made no change there either. Still behaves as described in OP.
Still wondering if anyone can solve this, or tell me if it’s time to reinstall retroPie or update to most recent version (I have no idea if that would help or just complicate things further). Would really like to get this 8bitdo controller working.