I have two suggestions that I’ve experienced. Both are rather old affairs and turn based.
The first is Darkwind. It’s still alive on Steam, though it’s probably over a decade old at this point. It’s basically Car Wars, where you build up cars, strap guns to them, form convoys and go shoot up other cars. You can also just do deathtrack racing (with or without guns) or straight-up arena brawls, as your whims dictate.
There’s a strong AI enemy component to it, but there’s also a chance you may bump into other player convoys and they may or may not let you pass, so your mileage may vary. You can do this coop, though, as my wife and I (and sometimes an extra friend or two) got a lot of mileage sending out convoys of just a few heavily-armed SUVs to hunt pirates.
There is, as your requirements stipulate, no meaningful plot to get in the way (though it’s been a decade since I last played this) so who knows? The drawback is that a convoy mission can take some real hours before there’s an encounter, but supposing you have enough cash, fuel, crew and cars, there’s nothing stopping you from sending out multiple convoys.
Going with what @Palooka just posted here, if you’re willing to make a bit of a compromise, I’m also going to suggest Megamek.
It’s a tactical game that you can play coop. What it is is an entirely standalone, thoroughly complete, tabletop simulator of the venerable Battletech game, except it does all the bureaucracy and accounting for you–and since there’s a LOT Of bureaucracy and accounting, this is an astonishingly large amount of time gained.
I don’t know how well it handles AI opponents. I think it’s possible, I’ve just never tried it. You can form “teams”, you can fight other players, and it’s all very network-friendly.
Drawback: You’ll have to handle the character progression yourself. You may want a GM to oversee the affair or something. I mean, it’s literally a Battletech tabletop simulator.
Other Drawback: Darkwind is rather old and the graphics are a bit basic. Megamek is “graphics are the fourth priority,” and the graphics are phenomenally basic.
Both are, at least, free to play, though Darkwind had a modest monthly fee to unlock extra features. So you can try either out for size and see how they grab you.