Thank you for the reply. The distance checking might be important, I've only used simple collisions (Area2D on bullets and CollisionArea2Ds on the KinematicBodies that are the player and the enemies). Though I do wonder whether distance checking is actually more optimal, considering Godot's own collisions are probably optimized in a similar way.
I can't recall right now how it worked on itch. Playing the project from the editor into HTML5 works mostly fine, though it definitely has some FPS drops that I don't see in the Windows export version. I do remember that when uploaded on itch, it worked so much more terribly worse than how I had tested it locally that it just wasn't worth leaving it up since it would leave a bad impression of the game.
I don't think I hit even 1000 bullets at any point in the game right now, though the bullet deletion is definitely a very quickly put together hack with VisibilityNotifiers on the bullets + a bunch of rectangular collision boxes outside the play area just in case, so maybe there are times when bullets aren't getting deleted as quickly as they should; I should change that to be done manually as soon as they exit the visible area of the game.
I'll check those links and keep that info in mind for next time, thank you.