go to shortcuts on your phone, make a new shortcut, and search for watch. Once you click on that, you'll see all the things you can set on your watch. Low power isn't one of them. There might be some combination of things that do what you want. Once something is in a shortcut, it's easy to add a timed automation that runs it. Used to do this to turn off "always on" overnight before I was using sleep mode.
From looking at what low power turns off, I think most of the things that Low power disables, also don't run when it's off your wrist. (and scrolling back up, Howard2k also had the same thought)
or just put a charger by your bed, and charge it every night.