Really clean, well-executed node-buster here! Amazing how different it can feel with a twist like placing bombs limited by distance from each other, it's more strategic. I definitely felt every upgrade, which is what you want. Very solid design.
This could really build out, especially with more sfx, vfx, music, weird variations on rocks (rocks that need more than 1 explosion until you upgrade power? rocks that explode and push your placed ones outward? rocks that ricochet and bounce around other rocks? upgrading individual bombs so your third bomb becomes special-er? bombs that explode in a plus shape?) Lots of ways to go!