Really neat game! I feel like I've seen a lot of letter based games for some reason, but this still has it's own ideas that work well. I like how the letters you collect always correspond to actual words. Stuff like the enemies giving you all the letters from their names and attacks, and how the chests give you letters in the form of names of items. I also appreciate you were committed enough to the typing idea to also use it for menus and navigation. It makes the game feel very cohesive.
The idea of damage based on the rarity is interesting and works well most of the time. Sometimes it's a bit unbalanced though. I started one run with some common word (CRUSH maybe?) and felt like I never had enough damage to do anything. The next time I started with SLAY, picked up a Y upgrade, then cruised through the game one-shotting most enemies.
Speaking of upgrades, I feel that could've been communicated a bit clearer. I assume you add the damage/rarity for each letter, and upgraded letters are counted twice. This leads to confusing scenarios where sometimes an upgrade massively boosts a word, and other times does barely anything. Maybe the ability interface could show how the damage for a word is made up e.g. SLAY(100) = S(10) + L(10) + A(5) + Y(75).
Regardless, this is still a very well made game with a clever concept!