So I just tested it before seeing your edit (me issue - it was right there). I realized a few things. It takes a second while the Godot logo video is playing before you can send the signal. (BTW spacebar and mouse click work, as does B on the Gamepad.) I tried to make it pretty straightforward. Bad problem - the spacebar will also trigger then game start - but then maybe that's ok for now. You can still bring the menu up.
I added it to my blog list for tonight, but you can also check out my Game Template code now if you're curious. It's in the GameStateSplashScreen code under the _input() function. You can follow the logic from there.
So I've been playing today with the controls. I increased acceleration, actually implemented max speed, and changed the shoot key to spacebar. I felt the same thing that it feels better there. I also left missiles as Alt, and knowing about Ctrl + W I will not map ctrl. Thanks for the info!
Next one will be later tonight I'm sure, but I'm glad someone is getting something out of them.