Had a ton of fun with this! The only clunky piece to it was the actual upgrading of the letters. Perhaps there is a natural progression that letters upgrade the more you use them? That would incentivize typing out as many words as possible. Perhaps a bonus multiplier that ticks up the more "perfect" words are input. Speed multiplier too? That would set this up to help people get more wpm and accuracy typing.
It became more and more fun as the words got longer but longer words also allow you to delete letters and get perfect scores. Maybe completing enough words in a row would allow you to "earn" a deletion in order to keep your streak?
Overall: Super fun, very juicy, and addicting. Does complexity take into account words that are not on the "starting" line A - L or just words that one would not use in everyday life? I found myself screwing up the last letter several times going quickly where the word is similar to something I use in my day job (and thus muscle memory took over typing out the word) but was off by a letter...
A "win" screen would be nice, leaderboard, etc.
All of this adds to the scope of an already great game so totally up to you. All in all, huge fan!