I managed to defeat the boss on the second try. Took me about 5 or 6 loops each time to get there. First time its bullet pattern took me by surprise but on the second try I boosted my damage and critical, observed its pattern and defeated it. Nice assortment of weapons and upgrades. I enjoyed trying up different weapons. Sticky mines are my favorite. Really satisfying to see a mine latch onto an enemy and then go boom after a few seconds. I also liked the design of enemies.
The gameplay is smooth and engaging. There are things that could certainly be improved some more. Here's a couple of thoughts:
I expected side guns to fire sideways but turned out that 3 guns are firing forward and one is firing backwards. Maybe experiment with a movement/firing scheme where each gun covers a 90 degrees field of view and you can choose between several tactics of what is prioritized when vehicle is orienting itself. Maybe even always keep a constant orientation so left/right/forward/back are always the same.
Visibility is a problem with the boss fight. When the boss goes into low part of the arena you have no way of seeing where he is or what is he doing. Not really sure how this could be approached in a better way. Maybe add a key to flip the view 180 degrees.
The gray floor grid is kinda dull. Something should be happening there or at least don't use this dead neutral gray like Windows 95 did :). Maybe the color could wary a bit each loop, or there could be some subtle animations.
Many enemies are too similar to pickups in color and shape. Pickups could be differentiated, maybe through animation, or some effect like glow
The cash counter increment animation between levels lasts for too long.
Last but not least: Make the explosions juicier :)
Keep working on it and improving it! It can be fleshed out into an excellent game.