Thanks for your feedback!
I could indeed make the grid a bit smaller and respectively numbers a bit larger. The original plan was that I'd have larger puzzles, but turns out even smaller ones were just about challenging enough, so it served no purpose.
I also considered a glowing effect for the NEXT button when available, but due to time constraints wasn't able to implement that. You are entirely right on the fact that it is not as visible as it could be.
Difficulty is a tricky thing. I did try to assess puzzle difficulty with some weighted, more or less objective metrics, e.g number of moves to solve, ratio of winnable states in relation to all states, total block count. Interestingly enough, it seems that it is easier for me to create difficult-to-solve puzzles than non-trivial yet easy ones. The question of metrics is also tricky, as there still are some puzzles which I think do not exactly represent their position in the difficulty-sorted list.