Seems like a good start to a game but still feels very much like an early alpha.
The thing I enjoyed about games like Star Fox was the fact I was constantly being engaged by enemies and obstacles on the screen, but with this there are long moments of doing nothing but just watching my ship move forward.
Also, in terms of interaction, it feels extremely limiting to only be able to move the ship but not control its speed, or roll.
Lastly, every shmup (whether 3D or side scrolling) I've ever fallen in love with has some kind of, at least minimal, power up system. Without a power-up system, I have zero interest playing through a shmup unless it somehow wows me with aesthetics or a story.
Oh, and as far as the graphics go, I understand going for the old-skool low polygon count vibe, but even those broke things up with color variation which I feel this game could seriously benefit from. It doesn't have to be textures, just some colors to really differentiate the player ship from the enemies from the landscape, etc. etc.
... and just my own personal aesthetic preference... I feel the music is extremely lacking and only adds to the monotony of the game play as it currently stands. I apologize if those were some of your tunez, but I feel like the game could benefit tremendously from something more consistent and engaging...
Check out
http://freemusicarchive.org/ as they have thousands of really great tracks if you need some place to pull from.
I think what you have going here is truly brilliant if you could push it further, and I understand you're already feeling burnt out on it, but I'd absolutely love to see you pull back on making more levels and concentrate on polishing up the game. You obviously have a great engine setup and a really nice aesthetic beginning... now hit it out of the park with some good polish!
