That's totally right, if you had that impression it is totally right that the introduction of those signs should happen earlier, I think the main problems are level 7,8,9 which don't have any of the more interesting mechanics, my idea was to include them as a sort of a challenge to check if the player "mastered" the basic movements without all the other mechanics involved, and after those it can move on.
The Levels on the other pages unlock even when you did not complete all the levels in first page though, did you get a chance to see that? I am curious how your playthrough went to better understand how we can improve the progression
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As for today, I have a lot of games to rate, have to rush and don't have time to test all levels for too long unfortunately. If you're willing to develop this game further, 1). you aren't tied to the theme anymore (though focusing on +/- helps to stand out above other sliding box puzzles a lot), and 2). you really need a proper testing team to tweak and arrange that many levels, with trailing mechaincs and concepts in consequent levels and gradual difficulty increase. Sadly I don't think I'm the best candidate for that, with all my own projects and busyness.
As for my short playthrough, the moment with "-" sign deleting part in the middle, and moment with using L shape to complete a level was "WOW" moments for me, good experience. If I ever find time to play more and have more feedback, I'll let you know. Good luck!