In this immersive mode make the experience and frame like the PWA except for some feature :
1. When the there is a single tab, tab bar should be absent.
2. Pressing F4 reveals the the URL/Address bar.
3. May be F2 and F3 can assinged to go back and forward in history
Have you tried the new fullscreen experience though?https://techcommunity.microsoft.com/t5/Discussions/Finally-here-the-Real-Full-Screen-Experience-in-E...
@HotCakeX I agree that the new full screen mode is very good. but if it gets into windowed mode instead of a full screen it would be better.
Well there is also the Focus mode, have you tried it? should be enabled using the flag "Focus Mode" edge://flags/
it takes 2 clicks to either go into or get out of Focus mode.
it currently doesn't have a shortcut key
but once it gets one, i think it will be similar to what you're looking for, that and the new fullscreen mode
for what it's worth, I hope they consider adding a shortcut key for focus mode while considering your suggestion too :)
@HotCakeX but its has got very large border same as the browser window. See below