Hiding the top toolbar would create a big visual inconsistency, and would require a new way to navigate since that's where the back button is.
You can change Repertoire > Layouts > [layout name] > Edit Details > Actions > Close Full-Screen Document to Song Completion. The song completion will be determined by the duration you entered for the song, the end of a recording playing during the song, or the end of an automation track playing during the song, whichever comes later.
I've been steering people away from the full-screen toggle for a while now because it uses system elements for showing PDFs or lyrics, and those elements don't like being resized after they're loaded. This has created dozens of bugs over the years and is on my short list of features I'd love to get rid of.
