There is no native option to set the Following page as global homepage, but you can "hack" it by manually setting the value in the settings table to /following. The problem with that approach is that guest users see nothing.
If you use this extension of mine, there's actually a secondary feature that allow setting the follow page as homepage https://discuss.flarum.org/d/25994-follow-tags-prompt . When that extension is active, it changes the Following page so that it shows the regular discussion list for guests instead of an empty following page.
