I highly recommend that you read Christie Golden's Voyage books that take place after the show. While they are not great, they do lead into Kirsten Beyer's Voyager books. That way you get the complete story. Also, check out the reading chart at https://www.thetrekcollective.com/p/trek-lit-reading-order.html to see ALL of the books you really should read. There's more then just Voyager books. There are things that happen outside of Voyager that are incorporated into Voyager that you really should read.
I loved the String Theory trilogy. It is set in the middle of the series, very strong and interesting stories.
Again, I’m not a VOY fan, and I don’t plan on reading more than one book. Heck, I’ve averaged one Trek book a year the past few years and none for a decade before that. Plus, I have about twenty-five others on my shelves at home and work I picked up in the last year to go through. I’m looking for one special taste of VOY that maybe will enhance my experience of the series, and that’s it. I understand as a fan you’re saying go for the whole thing thing. Way back when I might have argued the same for the DS9 Relaunch (no closure, still pissed), but I don’t enjoy the thing enough to do that. You might as well suggest I read the all of Lord of the Rings or Game of Thrones or the DS9 Relaunch. It’s not happening for me right now. Maybe in ten or thirty years.
Maybe the recently released Autobiography of Kathryn Janeway would be a good one-and-done? I am also not much of a VOY fan, but I quite enjoyed it. And it was written by one of my fave Trek authors, Una McCormack.
The Mirror Universe Stories tend to be in their own universe (they could take place in seperate Mirror Universe’s that are similar to the one seen in TOS & DS9, but different). We had the Mirror Universe banner stories that were issued from 2007 to 2017 that tie into the Relaunch. Then there was the Dark Passions duology in 2001 that’s different, as well as Shatner’s Mirror Universe Trilogy (1998-2001) that’s different from DP. Then you also have the TNG novel “Dark Mirror” where the Terran Empire never fell.
In that case I'd recommend Mosaic (about Janeway) or Pathways (about everyone else). That'll enhance your experience of the show and be fairly self-contained, and it's neat to read what Jeri Taylor had in mind for the characters' backstories.
If you are up for a short story collection, Distant Shores, the anniversary anthology, has some good stories in it.