Hard as I am absolutely not good witz puzzle-mechanics design. Everything is now just guesswork: I would probably build in some mild form of failure at the very early levels followed by much harder failure options later on. After the jam I would ditch the placement randomness and would offer "puzzle pieces" for creating explosion lines and eventually some special explosions but I just saw potential not that I would be good at designing a good puzzler ^^
But from what I've learned is that you get help when you ask for help. Join a developement community and connect with people, maybe find a (beginner) game designer that likes puzzle games.
Stay on the developement path and enjoy your time :) The only real tip I can give you: Make what you like, not what others may like.
Stay crunchy.