How to get a one-key shortcut for full screening an Application (not just maximize)
System Preferences lets you change builtin menu shortcuts to almost anything you like.  This screen shot is from macOS 10.13 (High Sierra):



Here, you can see that I've created a shortcut under the All Applications heading, so any application that contains the particular menu item that takes its window to full screen will adopt the new shortcut.

The name of the menu item—which must be entered exactly, including capitalisation—is Enter Full Screen.  As you know, its default shortcut is ⌃⌘F, but here I've changed it to just the equals sign key, =.

If you try to establish a new shortcut to be a single key, you'll often be denied by Finder's beep, alerting you to an invalid command or value.  The trick to circumvent this when creating these shortcuts in System Preferences is to hold down Fn and press the desired key, which will then be accepted.

You'll want to create a second shortcut to take your application out of fullscreen and into normal mode again.  This menu item is called Exit Full Screen, and by default, shares the same shortcut as its cousin, therefore it makes sense that your new shortcut should be the same as the one you set to Enter Full Screen also.

With the setup in place as depicted by my screenshot, a single tap of my = key takes me fullscreen; and another tap takes me back out into normal view.  Cleverly, testing this in Safari, it doesn't respond to this shortcut whilst I'm typing in this text box, which is how I am still able to print an equals sign, =.  Once I leave the text box and the focus returns to a non-input element of the Safari window, the shortcut becomes active again.  I haven't tested to see whether this neat feature applies to other applications.  I suspect it will work for some, and won't for others.