Voyager was thrown into the DQ weeks after the first encounter with the Jem'Hadar... it was in Season 3 of DS9 after episode 6.
Janeway at the very least would have had knowledge of the Dominion residing in the Gamma Quadrant... and knew them to be a threat.
However, the Defiant was already well in service by then and was designed to fight and defeat the Borg (per Sisko's own words)... and since Voyager was state of the art, it stands to reason it would also have much of the same technologies as the Defiant... but was faster, warp-wise.
The Defiants shields in the initial encounter with Jem'Hadar ships actually held up nicely, so SF obviously implemented those lessons learned from the Borg which likely was more effective against Dominion ships (the Dominion didn't have superior tech over the Federation really, just better Intel on the Federation and its technology from the ships they captured in the first 2 years of DS9).
Anyway, Voyager knew (or should have known) where the Bajoran GQ end of the wormhole was, but it had no guarantees it would have been there by the time they reached it, plus I would imagine that going straight for the GQ (where a hostile Dominion resided) was likely not a good idea.
If the trip would have taken Voyager same amount of time and if the wormhole wasn't there, or something else happened, then they would have to travel on their own back to the AQ from there again.
It was a better decision to just go back to the AQ directly from their original location and not take a chance with the wormhole.
But I take issue with the premise it would have taken them over 70 years to do so.
Mainly because according to canon data, Warp 9.9 = 4 billion miles per second, and that would result in 21 473 times speed of light.
Even Harry Kim stated in the early seasons 'traveling at relative velocity of 2 billion miles per second' - or 10 736 times speed of light - when discussing a plan to beam Chakotay out of a Kazon ship.
Even at that velocity, it would have taken Voyager 7 or 7.5 years to get back - the writers could have gone with that.
Voyagers maximum sustainable cruise velocity is Warp 9.975.
Past 9.9, each increment results in exponential increase in velocity, which would mean that Warp 9.975 would take Voyager about a week to traverse 75 000 lightyears.
And no, it's not far fetched.
Most Federation ships were never seen achieving Warp 9.9 (or even 9.5) sustainably (and by that I mean as a cruising velocity) before Voyager in the first place, and the damage Voyager received in it's initial pull into the DQ was extensive (possibly preventing them from achieving those speeds and forced them to use slower velocities like Nebula class ships of the time, while their 'maximum' sustainable speed was reduced to say 9.5 - as Tuvok mentioned in Threshold episode that Paris exceeded their maximum velocity, which was mentioned to be 9.9).
I realize the show hinged on the premise of the ship taking decades to get back, but for their technology and overall cruising speed Voyager was said to have, it would have been easier for the writers to say that Voyager was thrown well beyond the Milky Way.
Warp 9.97 would equate to about 2.748 million times speed of light...
The observable universe is about 91 billion Lightyears across...
Throwing Voyager about 192.398 million lightyears away would have resulted in a 70 year trip.
Now THAT would have been far better and more suitable for their technology and ability.
Voyager was an entirely new ship class, so it wouldn't be a stretch that Starfleet managed to make it a lot faster than all other ships in their fleet given faster than exponential developments and what happened in TNG with warp speed explosions in the first season (remember 'where no one has gone before'? - Enterprise-D was said it would take just over 300 years to get back from a distance of about 2.7 million lightyears away... that's about 15 000 times speed of light - at a velocity of Warp 9.2, or 9.4 at the time for the Enterprise-D - or maybe 9.6).
And we've seen the USS Prometheus (the experimental ship from Voyager's season 4 message in a bottle) that it was travelling at Warp 9.9 towards Romulan space - so it's likely that newest SF ships would be a lot faster - at least Warp 9.9 and above for sustainable cruising velocities.
I apologize for a bit of a rant... but thought it was relevant to mention.