This was fun and has a good base. It was pretty easy to figure out with a bit of experimenting on the first play, and I was able to win on my second try.
If you are going to have sound settings before the game starts, it would be good to add background music and SFX to the screen. For a SFX slider we usually play one after the value is changed. The menu SFX felt much louder than the combat sounds, and I'd adjust the volumes a bit so the SFX are all closer. I lowered the SFX volume down based on that menu sound, and then the combat sounds were too quiet.
The upgrade screens could use a bit of polish. When I first upgraded my weapon I wasn't sure about the options, since I thought I had a front weapon, but could still purchase a gun there, as well as anything else. I went ahead and got the machine gun for front, and it replaced the current gun that was there. Since it was an upgrade, it would be nice if you got a credit for your existing weapon. The play balance of costs didn't need this, but from a gameplay perspective, it is nice to get credit back for upgrades, even if it is only a portion of the original value. Similarly, it would have been nice to have a warning that your are replacing the weapon. Perhaps change the color of the buttons for mounting points where you already have a weapon and will be replacing it. Disabling the button for purchasing a weapon you already have in a slot would also be nice, unless you changed it so buying more of the same weapon in the same slot allowed for that weapon to be upgraded.
The boss fight was pretty easy. Avoid the things it spawns and try to get close enough to shoot. Certain phases felt like they were all about evasion, though at points I wanted to be able to shoot the spawns. Other phases were pretty easy to get in close and avoid the attacks while hitting back. I like that you give a heads up with the boss telegraphing moves, but it was perhaps a bit too easy. Maybe have the boss spawn some additional enemies, or add something else to mix it up a bit more?
I started with upgrading the front to a machine gun. Then added a grenade launcher in the rear spot, followed by 2 more machine guns at the sides. I upgraded health, time and damage with what was left over after weapons. Then I upgraded all 3 machine guns to lasers, and that was what I used to win.