Great game! I have a good deal of experience in bullet-hell games so I think I have some good feedback if you are intending to make this a long-term project.
Some aspects of this game reminds me a lot of another indie game called "Minishoot", if you want some inspiration consider checking that game out.
Some context on what I did during the run and what happened: It took me 2 attempts to get to endless mode, 1st attempt I focused on upgrading boosters, 2nd attempt I focused on pure speed.
What you did well:
Normally you'd have more enemies in this genre but in a game like this I believed you had the right amount of enemies.
The visuals look very appealing and it's good that you made the player/enemies/bullets contrast with the background well.
The movement feels smooth and has a good mixture of not punishing the player for making movement mistakes and at the same time not giving the player an absurd amount of freedom when switching directions etc.
I noticed in your itch page that you said you needed to decrease the difficulty. The difficulty in my opinion is pretty good, though I have some more comments on it below.
Suggestions:
Definitely having at least 1 boss or super unit would add some extra spice to the gameplay.
Additionally, you could potentially add structures. Here are some potential structures you could add:
- A satellite that moves very slowly and is very tanky but doesn't attack at all, and when its destroyed it gives a ton of XP compared to other enemies.
- A "booster" of sorts that buffs enemies in a certain radius.
You should allow the player a bit more freedom on what upgrades they can choose. I didn't play much of endless so maybe its different in there, but during the 10 min survival there weren't that many options for me to purely focus on a type of plane. Like when I wanted all speed, there were only 2 upgrades that gave me speed.
And speaking of upgrades, consider adding some other stats! Things like body defense/damage (triggers when you ram into an enemy but not a projectile) or piercing bullets can let the player customize a lot more.
Regarding the difficulty, I think its good but you definitely need to make it more scalable. The easiest way of this imo is to just have a slider that lets you control enemy stats, maybe a 0.5x - 3.0x slider for their hp & attack. There's a ton of other ways such as making harder difficulties have bigger enemy bullets/missiles or simply just faster enemies but regardless I highly advise you set up ways to alter difficulty.
Thanks for making this game! I hope you intend on working on this more.