coop is a must in this game if you want to progress like a normal person but the waiting time for your room to fill up is pretty ass. you can make a room while taking a shit and your toilet will fill up faster than your room
and soloing to grind is not exatcly an option because the AI in this game have a similar intelligence to a rock, you probably will have a better winning chance playing with a 4 year old instead because the AI just keep killing themselves like jihad squad
music is good indeed but only like 1 or 2 piece of them the rest are basically the same track with the same tune just with a different instrument or pacing
all in all, play this game only if you have friends to play this with. at least you can share the pain together. or really just play something else.