Agreed with Ryoga on the inertia - sorry to be so "absolute" about it, but it has to go. It works in games like Defender and Asteroids where it's a central gameplay element, but in a pure shoot/dodge'em up game, you want absolute 100% precise control of the spaceship. Galaga did it right, no reason to change that.
This might sound like nitpicking, but it's actually a big issue with anyone who's into that kind of games. It's a very common thing to add when westerners develop shooting games, and it's almost universally bad - a typical "euroshmup" trope. It makes the controls less intuitive and the game less fun to play, so why have it.

There are much more fun ways to make a game harder.
Otherwise I think this looks reall promising and a lot of fun.
Oh yeah, another things shmup developers learned over time is that the hitbox of your ship should never be larger than it needs to. I'm not talking bullet hell style 1x1 px hitboxes, just something that's at least somewhat smaller than the visible sprite, to make the game feel more fair to your player. Whenever something feels off about the hitbox, you much rather want the player to think "whew, I don't know what happened, but I was lucky I made that" than "what the hell, I didn't touch that!"
The latter was a stable of many otherwise great, classic Konami shooters, and they would have been better off without that (ie. see the awesome PC Engine port of Salamander vs. the problematic arcade original)