This is an awesome concept. I'm of big fan of games like Typing of the Dead and Bloodtypers and this already resonates that kind of feeling. I love the variety in words and the slight western theme that you have going on. Cowboy is my bane and has gotten me multiple times.
I had a couple of moments where I had two words show up with a similar starting letter or were even just the same word. The typing priority is a little wonky where if that happens its hard to recover.
In terms of difficulty the harder modes seem pretty tuned well. I love the words used in the harder modes, they really test your literacy on a keyboard.
Sometimes the enemies approach a little too fast which is always adjustable but I think this is a really solid approach to the genre.
Great Job and I hope to see this game flourish!