Bomb Radius up to level 26+ and the field is yours with one bomb. I like the idea but it needs a bit polish
I would "refine" the idea a little. This is just a short idea: first of all I would set a maximum explosion size (at lvl 12 or maximum 15). Set also a maximum lengh between the distance. Now add a new upgrade for (as example) shrapnels (smaller explosiv parts/fragments hurled away from the main explosion). If you are into SciFi for the game, you can also add "laser explosions", where a laser shoots along the fuse line and destroys everything on the way. That upgrade can basically upgraded from a single line to a "destructive wave", so the fuse is not just a lengh limit, it is also an active gameplay element.
I can see the potential in the game idea and it's fun.
I know your troubles. I'm working on a kind of retro game (I try to mix columns, vampire survivors and balatro) and I reworked the upgrades and mechanics like 5 times? Now I think it works somehow but I'm still not happy with it. And the worst thing about it is that my fixes are awesome for the gameplay but nobody will ever see the work behind it.