I mean, I think the answer here is that the writers nerfed the Borg severely in VOY. We could try to make some excuse about human ingenuity and instinct but, really, it's just because that's how they wrote the stories. ¯\_(ツ)_/¯ To quote Unimatrix Zero,

SEVEN: This is a class four tactical vessel, heavily armed. The central plexus is protected by multi-regenerative security grids. We'd be detected long before we could reach it.

JANEWAY: YOLO!!

It seemed to me that it was mostly a combination of Seven's inside knowledge and really good luck.

Plot necessities.

Seven was definitely an advantage, but the Borg did seem to be unnecessarily weakened in VOY. One of the general problems with VOY was the less convincing antagonists.

We should also consider that Voyager was a ship built after the Battle of Wolf 359, at a time when Starfleet had changed their approach to ship building so to better prevent such an incident from happening again and to stand a better chance against the Borg in future encounters. It would seem Starfleet succeeded better than anticipated in that regard. Add in the advantages Seven brought to the table (as well as a few other Borg-related upgrades garnered during the "Scorpion" two-parter) and just some good tactics overall, that would give her an advantage past Starfleet ships wouldn't have had before.

But yeah, even I will concede the staff gave Voyager an obvious amount of plot armor to still achieve these victories as well as she did, though I really only found "Dark Frontier" to be the most grievous offender of this--given the alliance struck with the Borg means "Scorpion" doesn't really count, "Unimatrix Zero" only for the fight for that tactical cube and that given how that was somewhat staged so to achieve an ulterior goal I'm more willing to forgive it, and obviously for "Endgame" Voyager had received enough upgrades to make its relative ease fighting the Borg pretty justified. Beyond those, any other Borg encounters by the Voyager crew were typically had a good reason why the Borg weren't bringing their A-game to the table.

Besides, when you really think about it, every hero ship going up against the Borg tends to have a higher amount of plot armor favoring them than the background ships would. I mean, the Enterprise-D still got out of its various encounters with the Borg a lot better than other ships did, even though she didn't really have any major advantages the other ships wouldn't.

^ Liked your post expecially for the last paragraph (but agree to the rest, too).

Plot armor is the main reason. And I don't find it offensive - it's a natural part of storytelling. Star Trek is a fictional story, and I'm enjoying it as such. It follows other laws than the real world, and that's how I like my stories. The heroes will normally win, and this is a good thing (and if they loose, they do so for a dramatically engaging black ending).

Having said this, there's a tendency over the course of Star Trek to integrate former enemies. The Klingons were stereotypical enemies in TOS and became allies in TNG. The Ferengi were stereotypical enemies in TNG and became relateable fellows in DS9.

There was one Klingon crewmember introduced in TNG.

There was one Ferengi station inhabitant inroduced in DS9 (well, in fact, there was more than one, but by all means, Quark was the typical Ferengi of the show, the other recurring Ferengi characters, by the time they really got fleshed out, were used to transcend the concept of Ferengi).

The Borg were stereotypical enemies in TNG and DS9 and then in VOY the authors obviousely wanted to integrate them, too.

It was not as easy as with the Kllingons or Ferengi, who are basically metaphors of human character traits, the Borg being the biomechanical hiveminded, deliberately inhuman classic scifi enemies they are, but the authors tried their best.

They found a way to integrate a Borg crew member, and found ways to make negotiations with the Borg possible, which contradicts their very concept, and become at least temporary allies.

I think there is a pattern behind this.

The whole Good vs. Evil paradox. Good wins 90% of the time and it was used to the advantage by the writers when it came to the Borg.