Thanks for your feedback! You're right that there's one clear strategy, which makes the challenge to come up with one good word per level. My feeling right now is that it would be more fun to make doing that more challenging, rather than punishing it or rewarding shorter words, because shorter words are easier to think of.
Another take is that the game needs to make better use of each level, rather than asking you to do just one thing. Maybe I should make players collect keys rather than spelling words of a minimum length.