The plist was a copy of homebrew-update, label included, so launchd never loaded it as a job of its own. Replace it with an osascript job that runs at 1:00 am and, for each of Chrome, Vivaldi and Floorp that is running, quits it the way Cmd-Q does, waits up to a minute for it to exit, then reopens it in the background. A browser held open by a quit prompt is left alone and logged.