I actually had it on half-opacity after I was alerted to that. To answer your question, the full is dark enough. One thing you can similar to games with “Brightness Check”. Since label splashscreen is like any other label except it plays first, you can use it to show a screen. Maybe a screenshot with text textbox opacity at 0, 50, 100 (I think it would be 0, 0.5, 1 in Renpy), and each one would set an initial value for players, and they can adjust it later in settings. :)
Oh! I guess that's like in some games, they ask players to adjust the brightness till they can/can't see a text anymore, but I do this for textbox opacity so that players can set their preference before starting a game, and it's also changeable during the game play! :D
Thank you so muchhhhh for your suggestion! :3