I do think the difficulty curve could have been a bit better, for example level 3 is probably the hardest in the game for me. The solution is very cool and clever but I got stuck on this one and had to cheat. Generally, longer words are harder.
With that said, I'm kind of unsalvageable on this kind of game, my brain simply can't do anagrams. The friends I've send it to, whom are expert wordle-er, have found it very enjoyable :)
Also, I was wondering if the flag / goal difference was an oversight or intentional design ? If intentional, I don't think it's good design to have the player do two different things for the same desired outcome.. Either introduce the goal solution in a level that doesn't have the letter "f" or allow both to work.