Bombs dropping from bosses would probably be a more appropriate way to return bombs than basing it on kills since midbosses will drop bomb pickups in the mainline series. The graze mechanic could also use a bit of visual feedback -- to that I suggest using the rad pickup sound in conjunction with spawning stress particles around grazed projectiles.
As for the boss, I enjoyed it, but the difficulty is all over the place with certain attacks when you're playing ranged because of how quickly the projectiles come out with no tell. The rad pickups also get stuck on the invisible walls, but that's a base game problem.