First up,
CounterAttack Uprising, or just CounterAttack as they've slapped on the "Uprising" after some update. It is lacking in visuals, sounds, music, but its weapon customization is quite extensive, even if a bunch of attachments that affect your stats or weapons in the advertised "over 500 attachments" are just same thing but with +5% more damage. Some of the attachments also can change your weapons into something else, like turning basic shot into a flamethrower, add homing onto missiles and etc, and some of those upgrades can be mixed, like combining MIRV missiles with homing ones to spam up the screen with them, or making the spiral flamethrower shot.
So, with all the customization, player power can get ridiculous during gameplay, but one would have to get to that point in the first place by upgrading weapons through Gradius like system, where you have to pick up a select number of upgrade powerups and then press a button to claim the highlighted upgrade. All upgrades have a cap depending on the ship you've chosen, but the "add-on" upgrade, which costs quite a bunch of powerups, ups the cap by 1 each time it is bought... Although I feel like I may be rambling too much, but I hope this gets the point across that I may be too autistic about making builds to destroy things with.
CounterAttack also has coop online multiplayer for up to 8 people, though lobbies there are mostly dead, and it seems like to play with someone you gotta get on their Discord. Typical.