This is a cute concept. I really like the idea of having to budget lives which you are also using as a resource. I didn't realize that the timer was a fuse until I came here and saw the other comment. I even bought some of the fuse length upgrades and couldn't figure out what they did because I was assuming that the time after I pressed space was the fuse. I would occasionally blow up for no apparent reason, and maybe that was always because of the fuse timer but I think I saw it happen when the timer hadn't finished yet.
The knockback from enemies felt unnecessarily punishing. For me it made trying to set up multiple enemy kills just frustrating. I think removing the knockback completely or letting the player move, maybe only slowly, after triggering an explosion would help.
Some of the longer, maze like levels felt pretty slow to me. I think I would have preferred more short levels.
There was some kind of bug around invincibility time after being hit. I would sometimes explode after an enemy touch me and stay the dark color and invincible forever afterwards.