Thank you for your feedback and im glad you enjoyed the game.
1) There actually is invincibility after getting hit. Maybe i should extend it from 0.3 seconds to 0.5 +- seconds, or show it with something like shield.
2) From other comments re balancing enemy spawn would help. They probably should spawn around level 5/6 and not at level 3.
3) Should have added check if they can spawn, hopefully it didnt ruin your run.
4) I didnt think of that, i have notebook monitor so i can see them well and im glad you pointed this out for my future projects.
5) Yes more upgrades would help and some re balancing should be done, but i wanted to have some busted upgrades (like fire rate) so you can feel diference when taken. Interesting idea with max health and ye it would probably be better.
Once again big thanks for this extensive feedback. :)