(for context: my favorite game is monolith and if you don't know that, it is a twin-stick shooter rouge-lite with a weapon modifier mechanic similar to your combining bullet mechanic)
I really Enjoy the game, especially the bullet combining. I does provide the same feeling with monolith once you found a gun with OP modifiers. I just wish it was longer with some more tanky enemies since combining all absolutely shreds everything.
Like noeticnightjar said, the game could use some balancing. the boss don't stand a chance if you combined all billets and you won't stand a chance if your bullet combination does not have enough DPS. maybe you can try adding a bullet combination limit to 3 or 4. Either way I wound be happy to see this game gets expanded (or turned into a rouge-lite), also I second Karod's laser bullet suggestion.
if you want bullet modifier reference I highly recommend checking out monolith, it is such an underrated rouge-lite