Super enjoyed this game! I think it would also work as a full game with extra puzzle mechanics.
I wrote out a long comment a couple of days ago and I see now it didn't get posted but the base of it was similar to xeno's comment. I agree that it could easily be a 20 second game. It's OK for a game to be hard as long as it's rewarding when you don't reach the end. It can be good to give you room for improvement. My thought was that you could zone the letters with four to six different background colours. People who type fast can aim to finish. People who type more slowly can aim to get to the second zone and from there try to push themselves to reach the next. (Just a top of the head thought).
Repeated letters: my first thought about this was that it should be possible to eliminate them from the grid so the player doesn't fall victim to an unfair grid (I got into a situation where I was surrounded on all playable sides by the same letter). My second thought was that it adds a puzzle dimension to the game that you could exploit to make the gameplay more challenging. Maybe a hard mode where you ensure that repeated letters _do_ happen (but that there is still a way to get around them). Could be fun for a future iteration.