Nice game! my only recommendations would be to be able to press the start button with the keyboard, so you dont have to grab the mouse and click it every time.
you can do this by adding this code to the _on_ready function:
$StartButton.grab_focus() #replace StartButton with node name
or by just checking if a button is pressed
and also make sure the spikes dont spawn on you(you probly already did this) (and maybe give a little animation. and also as a rule of thumb to make the spikes hitboxs a bit smaller