Such a great realization of a concept that is almost too simple to be fun, but it definitely works! It feels like Puzzmo's Typeshift & Wordbind where it seems like you're gonna be doing the same thing over and over, but each new puzzle feels fresh and challenging.
I do think it leans on the non-repeating words too much for difficulty though. I got to the final challenge and couldn't beat it. I also think I'm dumb or missing something because I only got 2 vowels, but I see that others have more. (Maybe it's random and I just got super screwed by getting VWXYZ and only 2 vowels?) The final requires you to use a 2-letter word, and I'm pretty sure I don't have any options for that anymore, especially since it's attached to a long word. If I knew how critical 2-letter words were, I would've saved a few of them in my back pocket. But there's no way to know that you'll need them later.
The open world is very cool to explore, but it did make me unsure whether I was missing something or missing a letter. I scoured and rescoured the world several times to find another vowel and couldn't find anything. I'm not sure if there's some non-telegraphed puzzles hiding letters (dung beetle maybe?) but in that case, I would still feel like the game is trying too hard to keep vowels from me.
Idea to make the non-repeating limit softer: Have an energy meter that is built up by making unique words. Repeating a word costs some multiple of the energy that it rewarded. So if SEWER gives 5 energy then reusing it costs 25 energy, so you can tune the ratio of unique to repeated words that way.