5. and 6. The problem with having settings like that is that they have a substantial effect on gameplay. That means, that there's an optimal setting for this game, and that any other setting is now useless to people who want to play the best they can. If you can adjust darkness levels in a game that's partly difficult because of the limited visibility of darkness, why wouldn't you just set it to be as bright as possible? Similar problem with audio. If there's music that can make hearing the enemies harder, every player who wants to play the best will just turn off the music, but there's people out there who would feel like that's "wrong", and not the intended way to play the game. You would then have people playing in a sub-optimal way, and potentially not enjoying the game to its fullest, just because they have the option to.
10. Having huge big chunky bullets that are clearly visible on the screen is generally very satisfying, have a look at any game by vlambeer to know what I'm talking about.
This game is quite similar to Devil Daggers, if you've ever heard of it. If not then I suggest you check it out when you get the chance, it's available on Steam and GOG. Another thing I think you should look at is the video Matthewmatosis did on Devil Daggers, that I have linked in this post. I think you could learn a lot from this amazing review (and the entire channel in general) and Devil Daggers.