Thank you for your godlike feedback! :) I really appreciate you attention to details, i thought that i'm doing big texts, but you are just on another level :)
I value your tips about ui and art part because i'm not artist myself and just trying to follow common sense, these can help me to actually improve it more. Btw space theme was kinda happy accident, at first i was looking for foresty-mountiany background (just because i prefer that and i have arena somewhat castlelike) but could not find in the same style of art i've already had, but then i found skybox and used its textures for backgrounds, lol :)
Single player is noted, that's at least more appropriate thing for game jam. I'm with you and @DevaosX in the sense that i also sometimes lose to non-moving couterpart, so annoying, haha :D
"Score seems to go below 0 (currently at -2.5)"
Do you mean health or rating at Leaderboard? I'm assuming health because for rating i would be really surprised for this result because it demands a huge amount of games and even then it should not go below 0
Sound would definitely help, agreed.
About momentum, bump, bounce - ye, it still feels clunky to me, good to see another opinion on that. Wall bounce was another happy accident btw, that was a try to solve bullet reflection problem, but i did it other way, but i enjoyed bounce so i left it :)
Addition of good items also sounds like a good idea, mb it will be better soultion to force players moving instead of just bullets as it was dicussed at other comments