There are no plans to use SLS for national security missions. SpaceX, ULA, and Rocket Lab all do national security missions, and hopefully Blue Origin can join in as well in the next couple years.
In a sense, all the ostensible science missions are really about national prestige and are therefore national security missions in a round-about way. I saw a recent presentation given by Michael Griffin in which he makes this point; the HST wouldn't really be worth all the trouble if not for the "America flexing on the world" angle.
Sure, but who’s actually launching those science missions these days? It’s all commercial providers. Recycling 1970’s technology into SLS and making it even less reusable isn’t a flex on the world, it’s pork barrel politics.
I share your thoughts towards the lameness of SLS, but consider: would it even be happening if not for the threat of China going to the Moon? It seems to me that the whole point is to spoil China's fun.