I can't recall the exact where and when but I'm pretty sure the Borg encountered Seven Of Nine's parents before they encountered the Enterprise D. IIRC I think the Borg had an encounter with humans during the show Enterprise as well, which meant it was before even Kirk's time.

Yes they found Anika Hansen and her parents, which no doubt would have informed them of humanity's location etc, but according to other episodes that wouldn't necessarily have sent the Borg heading to Earth at that time. Especially if the technology of the Hansen's ship wasn't sufficiently advanced to get their interest.

And as I recall, the Borg from the "Enterprise" series (which contained a whole lot of other retcon) were likely leftovers from those who traveled back in time from the "First Contact" movie. (Not much other reason for them to be there.) If they hadn't been destroyed they might have been able to contact the Borg in the Delta Quadrant at that time in a manner similar to what they were attempting in the "First Contact" movie, which presumably would have arrived much quicker. And a specific message like that might have done more to bring Borg attention to Earth than the essentially random encounter with Anika's parents.

So much of the "Enterprise" series especially was made up of "memberberries" that it made for a lot of continuity problems with the other series-es.

Not the least being, why wouldn't Picard and even Kirk have already known about the Borg?

Yes I think you're correct - the incident in the Enterprise series was attached to the movie First Contact, which does kind of mess up how Kirk and Picard don't even know about the Borg in the first place. Different timeline, I guess? As for whether or not humanity's contact with the Borg improved things, I guess you could say it certainly improved weapons and defences. We can see this first hand in the Voyager finale with the phasing torpedoes and heavy duty armour (although exposing the Borg to future tech, allowing them to "adapt" to it, wasn't such a smart idea IMO).

I had thought there was something more specific in the first Borg episode - "Q Who" - about how "early" that encounter came. But apparently not. Or I just can't find it again.

Are you thinking of a conversation between Guinan and Picard? I'm not sure it was in Q Who, it may have been in Best Of Both Worlds.

Without the trans-warp corridors etc, destroyed at the end of Voyager, the Borg were... severely disadvantaged. Probably delayed even moreso than if they just hadn't "felt" any urgency in following up on what they learned from the Hansens.

You gotta wonder, though, why they didn't do more and sooner considering that in the finale of Voyager we learn that there was a trans-warp corridor portal less than one 1 light-year from Earth. I don't recall any evidence that the portals were "moveable" so it must have been there for a loooong time.

Yeah I remember thinking the trans-warp corridors we found out about during the Voyager finale created more problems than it solved and for just the reasons you listed. I don't know about anyone else but it's tricky for me keeping straight just which trans-warp goes with what and which works and which doesn't. It seems every time Star Fleet attempted trans-warp (The Excelsior from Star Trek IV and Voyagers attempt which turned Paris and Janeway into a couple of catfish) it seems to have failed. I know the Excelsior was sabotaged by Scotty but I'm fairly certain I remember someone in another series discussing the Excelsior and how Star Fleet never got trans-warp to work.

Yes, and you'd also figure they would have proven it out before building a whole starship based on it.

I suppose maybe it worked on a smaller scale but not for a whole starship or something. Because Plot/Because Drama.

It could also be argued that "trans-warp" just meaning "beyond warp" there might be multiple theoretical ways to achieve it, and the Borg used something different. Their "hubs" were apparently powered by stars, after all, perhaps entire nebulas.