Hi,
TL;DR: Aqara Zigbee Curtain Driver E1 Zigbee factory reset is a TRAP. Keep the button pressed until you hear TWO beeps to re-pair with ZHA.
After another 2W of fiddling, the Aqara factory reset doesn’t work as described in the manual. After some further HAOS15 / ZHA issues, I couldn’t get either of my curtain drivers to work, so followed the “press for 5S” instructions for a factory reset. After this, the ZHA pairing did not complete - just sat there interviewing.
It appears a factory reset ISN’T.
You need to keep a button pressed for 5S to get a first beep, THEN keep it pressed for a further few seconds to get a second beep.
Only then will it re-pair with ZHA. 
Hope this appears with a forum search, and helps someone!