Very tedious and repetetive
This game has OBVIOUSLY had many hours of hard work put into it, as can be displayed by the quality of the graphics and the programming.
HOWEVER
Graphics arent everything. As far as the first two levels went, they were VERY tedious and VERY repetitive. Not just because they were long, but that it took about 5 shots to kill an enemy craft, that your gun was SEMI-AUTOMATIC, that it was simple to mistakenly change back to the beginning gun of the game, and that each level (atleast the first two, but since they were only 1 and 1.5 stars I can assume the rest are the same) lasts roughly 5-8 minutes. The bosses are also very repetetive and easy to kill - basically you're SO fast that they can't hit you, but they have 9999 hp and 20 armor, while you're doing 30 damage.
I also think you should re-think your strategy about difficulty. Just because a level is looong does not mean it's difficult. It means that it's long, boring, and tiring. You should make the units have varying attacks rather than increasing the amount of them and their HP.
Also, about the firing button: how many side scrollers do you see today that DONT have automatic guns? This was done for two reasons. 1. People like automatic better. 2. It is much less tiring on the hands. and 3. IT GREATLY REDUCES THE CARPEL TUNNEL SYNDROME OF GAMES. Depressing a button for 10 minutes takes almost no muscle movement after the beginning effort and is very simple. HOWEVER, tapping a button requires alot of muscle movement and wears out the condition of the muscle.
To conclude this, it's obvious that you put alot of time into this game, and if you don't want to change the level layout due to whatever reasons you have, atleast give the game an automatic weapons system.