Indeed the close deadline of the jam prevented me from balancing the levels' difficulty; sorry about that. Will definitely engage in that.
Yes your goal value idea is good, and your max move count is excellent; will definitely incorporate them. In fact, I'm starting to wonder if the pause mechanic actually defeats the point of the timer: I put the latter there to add an extra challenge to make the player think quickly, but all that can be annulled with a press of the key F (pause button). But your comments about pausing's utility are interesting. What do you think about this?
Thanks for your detailed feedback!