For those who don't mind "full screen" still including the top bar, I find https://www.hammerspoon.org/ to be a good compromise. I set up option+shift+f to full screen my window instantly.
+1 for this. Hammerspoon is a godsend. I have keyboard shortcuts to switch to my desired apps instantly (around 10 or so, hyper+I is IntelliJ, hyper+A is Atom, hyper+F is Firefox etc.)
If the top bar is still there, is this different to the built in "double click title bar" maximise?
The '+' button and double-clicking the title-bar doesn't maximize windows—it zooms them in Apple's terminology. It should just make the window big enough to display its contents, although some apps don't cooperate (Chrome). Hammerspoon can be configured to maximize windows, i.e. make them fill the screen.