First off... I think I spent about half an hour just playing this game! So either I find typing surprisingly fun or you managed to make something really fun, which I highly believe is the case!
About the controls... It's a shame Godot can't detect wether uppercase is enabled or not. I'm used to typing uppercase letters by using caps lock instead of shift. Also, maybe this one point is because I'm too used to monkeytype, but I think errors should not progress the line or backspace should actually work, so the player is forced to type it perfectly, because I found myself getting unecessary, consecutive mistakes after the first one - either because I wanted to fix the mistake or because I tried to type the correct letter when the game was already expecting me to type the next one.
Lastly, I think I bought, like, 40 mutations that slows down the text, but I felt like nothing was changing, so I was stuck at a maximum survival time of ~40 seconds, which was kinda frustrating, ngl.
Other than that, I found this game extremely fun! Nice job!