that last storm is NOT a vision. how is a vision being reflected on the god damn glass of the sliding door? to be more accurate about its vision-ness!?!! no

all the symbolism points to the reality of the unavoidable storm/doom being away from his home:
-the black birds in an odd formation: symbolic of travel/migration
-the strangers who grab them in his nightmare from the car: symbolic of again, being away... people (or place) he cannot see or know harming him and his daughter
-his wife wet in the kitchen near the knife: symbolic of her finally being "drenched" in his trauma and experience. the knife is not the harm she is going to inflict on him, but the harm they may both face

that last scene is not about his wife finally being with him, she was already "with him". take for instance the the scene at the community dinner. most wives would lose their shit, but her expression showed she was beginning to realize he was going through something very profound, and she needed to be by his side. By the time they take shelter she agrees to the gas masks and tries her best to comfort him and their daughter. when she asks him to open the shelter, it's also about him trusting her and the daughter, so that they are all in it together.

by the time they make the trip they are one unit, but the dread he was sensing--that was always about harm beyond his self preservation, comes. like you said earlier the over-arching theme is that some shit is just unavoidable. i also think the film is an indictment of how and what we classify as a person being mentally ill/disturbed, but that's another conversation