Thank you for taking the time to test my game and for your feedback.
Regarding adding a weapon to an equipped weapon slot:
If the chosen weapon is different from the equipped weapon, the new weapon replaces the old one. Otherwise, and in any case, the game takes your money :-) It's clear that the game shouldn't let you pay for a weapon you already have.
Sorry that your boss fight was boring. I can confirm that with other weapons, such as the laser ;-), things are easier. And/or mines that can be placed in the boss's path.
Nevertheless, it is possible to shoot at the boss with machine guns in each of its phases, even when it moves in a lemniscate shape. Upgrading the vehicle's speed greatly helps you get to the right place at the right time.
That being said, it would be worth adding an upgrade to increase the overall range of the weapons.
Thanks for the suggestions for the sound and visuals, which I will be sure to follow up on soon ;-)