Exiting
Having noticed that Alt+F4 doesn't really close Opus (it's still there in ProcessHacker), I tried to assign it to
Close PROGRAM
Customize / Keys does not show a conflict with this binding, but dopus.exe still doesn't close. Other combinations work, however. Is Alt+F4 reserved, so that I need to pick another combo?
The how to exit guide has various ways to stop and/or restart Opus, to suit different needs.
If you want Opus to exit when there are no windows open, there is an option for that (shown in the guide), although not having Opus in the background will lose you some features (e.g. system-wide hotkeys) while it is not running.