That was my initial plan but there is limit to how many keys you can press together on a keyboard actually (you can search for keyboard keyroll test in google) I chose those specific keys because they feel natural but most importantly you can press all non-thumb keys that is qwer and uiop together and all of them is inputed to the operating system at the same time. (although I am not sure and dont think godot is capable of it)
your idea is actually possible but I have to map the thumb keys to left alt and spacebar. but in most people's system it is mapped to something.
and holding keys does feel good but it will be cramp inducing after some rounds. At the same time, It will be super easy to misinput.
I already tested it in earlier version of the game.