Adding something using the “the_content” filter is very tricky (maybe if you’re really good at writing regular expressions you can do it). I can just add the “read-more” link to the end of the content. But then I have two paragraphs at the end, the last paragraph of the content following by a paragraph for the link and this ends up having the “read-more” link on a separate line. I could do that without using the filter by just using the action hook to put output the link after the content. The whole point of this exercise is to get the “read-more” link on the same line.
Maybe you could add an option so that it doesn’t pass false as the first argument?
Why do you try to stop other plugins or themes from using filters on the more link? You don’t do that in the GeneratePress theme code!