I think I'm the target audience for this game. Maxed out all the upgrades and wanted more. Overall this is a solid S-tier in my tier list, definitely 5 stars for fun. I'd really recommend adding upgrade descriptions to the game page, because there's some ambiguity. For example: Spawn rate — what does this mean? Do they spawn faster so I can get more points, or slower so I can catch my breath? Spawn delay — is this the delay after an enemy dies before it respawns, or the initial spawn delay at the start? Velocity — whose velocity are we talking about? Mine? Theirs? Do they move slower? Do I move faster? And so on.
The difficulty curve at the start is a bit rough — for the first couple of minutes I had no idea what was happening or what the rules were, I was just getting wrecked. But I'm a hardcore player, I like that kind of thing — I just worry a start like that might scare off more casual players.
Funny thing — you and I, fellow jammer, made almost the same game at its core: you shoot at enemies in a cone and dodge them, then upgrade using resources you get from killing them. But yours came out better from a design perspective — the quintessence of this idea. I tend to spread myself too thin, stuff in all kinds of unnecessary things and can't stop myself. Bravo, hats off!