Finished volume 3 yesterday. Really enjoyed it.

I still don't think this will be too hard to link up to the main story.
There were actually a lot of little throwbacks to the main series in volume 3. One in particular that stood out to me was the mention of "next Christmas", considering that by the time next Christmas rolls around, Kirito will need to have split up with Asuna over something, gotten involved with the Black Cats, and gotten them all killed.

I expect sometime in the year leading up to that, Heathcliff will make his appearance and start the Knights of Blood guild, which will somehow lead to Asuna and Kirito fighting and splitting up. Story-wise, this would probably make sense to happen around the 50th floor, which I think is notable because a ton of people are supposed to have died in that boss fight.
That's probably a long way off, though.

I'm hoping to see more of Morte and the beginning of the PKers and how that all turns out.
The interactions between the Dragon Knights guild and the ALS are pretty interesting, too.

Also, Argo is great, and it looks like she's even on the cover of volume 4, which just came out in Japan last month.