You just spent an entire novel—maybe years in the making—inside a character’s head, channeling her voice. It’s no surprise that trying out an alternate POV was jolting. Thing is, it’d be just as jolting to your readers. It’s for their sake that I recommend against a mid-series shift from first person to the all-knowing omniscient. (That’s what I’m guessing you meant by “a narrative voice.”)
Handling off-stage action is the biggest limitation of first person POV, and you don’t sound particularly wedded to that POV in Book 1, anyway. Try recasting Book 1 in omniscient for series consistency, or in a mix of third person and omniscient (third person being more amenable to such blending than first), as in the vivid and complicated His Dark Materials trilogy. Just do so with this in mind: Giving up the first person POV doesn’t mean giving up immediacy or emotion. Perhaps your discontent with the alternate POV stems not from the shift but rather from your rustiness with techniques like ‘Show, Don’t Tell,’ and using setting/props to influence/reflect characters’ feelings, and choosing dynamic words and sentence structure. While you’d no longer have direct access to a character’s thoughts and feelings, mining those techniques to their fullest would make your non-first person narrative just as immediate and emotional.
If first person still seems a must for your series, then tricks for Book 2 would be to bring in ancillary characters that can update your protagonist on outside events, or to have interlude chapters or scenes that use omniscient narrative. Both are common in thrillers, mysteries, and fantasy. In the end, you may need to accept that there are things you can’t tell your readers. They’ll be in the dark with your protagonist, reacting and piecing things together with her. But who says that’s bad? Stumbling through the literary darkness with a hero-in-the-making sounds pretty fun to me.