You can buy everything separately. If you just want the game, that's not a problem.
That said, it's worth noting that if you have an active subscription, you get 10% off on any game/figure purchases. Another way to think about it is if you buy a one-month subscription and then the game, you can get a 45% discount on your first month. That gives you access to the entire
subscription library for a full month (books/doujinshi/games are sold separately), and you can cancel the renewal of your subscription at any time. You can check out the
FAQ for more information.
But once again, if all you want is the game, you can buy that by itself.