I have seen a similar game idea a few times now, but this is by far the cleanest/most polished of them. I really like the sound design as well - really brings the experience together.
I think only weak part is the level design. Its not terrible dont get me wrong just very random and without a clear theme (could just be me). The last level is the only one i would really change as the timer per shot is excruciatingly long and you cant really predict what happens next anyway (as the small targets disappear). Id make the timer shorter and remove the targets along the way.
Another quality of life feature for the earlier levels, where you place targets not lines, would be to have the spawn location exactly on top of another one that is in the process of vanishing (if you place it near it). That way we could get some well needed determinism into the system (also the cannon could be synchronized?).
That is quite the lot of nitpicking - its just that all the good stuff was already mentionend xD
I overall liked it really much - neat concept.