This option is really only to handle external links that have a trailing slash to be redirected to the URL without it. While Grav ‘supports’ URLs with a trailing slash, the default URLs do not have them.
You would have to disable the Redirect trailing slash option, and then at the end of all your URL links, add a \ manually.
In other words the default Grav setup is great. As long as you don’t link to the page version with the trailing slash in your backlinks (which will hit the redirect anyway if you leave it on, which you should) then it’s just as strong as having a trailing slash.