Hey, currently I don't see why it would be necessary to add a button to exit the game, because on Web you can just close the tab, and on the standalone there's already a close button in the window running the game, unless you're playing fullscreen (is that your case?).
I'm planning to develop this game on my own pace (which is admittedly very slow) with no set release date. I don't currently plan to make Helixteus 4.