Hi Ian!,
Thanks for playing the game!! I'm glad you liked it!
I understand that it can be tiring to shoot for so long by hitting the space key, on the other hand it could be boring to leave the key pressed ... In the next update I will solve this problem by making the space key fire two shots in a row and then have to go back to press space to shoot. In short, you have to press the key half times to get the same firing ratio.
Thank you very much for your comment, it helps me a lot to make the game more playable! :)