Good idea, it has potential!
Here's an idea to improve the user experience, see what you think.
I would try to make the spawn point of the monsters change to the colour of the next monster, so the player could have a little more margin.
Un saludo ;)
Lol, thank you very much for the tribute.
This is a very personal opinion, and it's possible that the approach may not align with your vision. If you don't want to proceed in this direction, I would completely understand...
...here's my suggestion: if it's a puzzle game, I think it would make sense to be able to see the queue of monsters to appear (all of them, not just a hint like it is now). It would make it more of a puzzle and less like an arcade game.
Un saludo!