From a canon point of view?
Its possible the Borg might just want to occasionally 'prompt' the Federation by sending one ship at a time to the AQ and test their technical capabilities.
When Q first introduced the Enterprise-D to the Borg, the Enterprise managed to inflict heavy damage onto the Borg cube.
Each phaser beam inflicted 10% damage on the cube and had Picard continued firing, he probably could have destroyed the cube which might not have had enough information or time to adapt to it.
It wasn't until the Borg beamed into Enterprise main engineering and started downloading their data files that the Borg managed to adapt.
Also, we were lead to think that the Enterprise-D encounter with the Borg happened much sooner than it should have.
Had the Federation not encountered the Borg and continued to evolve, it might have even become technologically advanced to the point where their first contact with the Borg might turn out differently (in Federation favor).
What was it that Guinan said at the end of 'Q Who?'?...
'For right now, you're just raw material. And since they are aware of your existence...'
If the Federation was closer to the Borg (in galactic terms), it is possible that they might have sent more ships to assimilate the Federation... but as it stands, the Borg might just be sending a ship at specific intervals to see the rate of technological progression and adaptation of the Federation.
From what we saw of some Delta Quadrant species, technologically, they weren't overtly impressive.
And also, if the original timeline in which Voyager took extra 16 years to get back home (resulting in a total of 24 years away from the Federation), they managed to develop really powerful technology to keep the Borg at bay in that timeline.
As it stands, due to bringing Voyager home earlier, the Borg did manage to assimilate the armor technology... but not Transphasic torpedoes it would seem (you'd think that it would be a good idea to erase that and some other crucial information and technology from the future shuttle and admiral Janeway memories - but for all we know, admiral Janway might have made a copy of the shuttle's database and gave it to Voyager just in case).
At any rate, since the encounter with the Borg (and if admiral Janeway never changed time)... the Federation would have sufficiently developed to defend against the Borg in about 28 years total.
It is also possible that if Q never interfered, the Federatio would have developed these technologies or different (but equally effective ones) either way.
But, as others mentioned, efficiency is one of the things Borg value a lot.
So, sending dozens of ships halfway across the galaxy to assimilate the Federation doesn't seem too efficient - especially if you wish to prompt the Federation to develop better technologies which you can assimilate or adapt to in the interim.
Although... I never took it as realistic that it would take very long for the Federation to adapt.
This is where lack of using their powerful superluminal computing power was wasted.
They could have used various verbal commands to the computer and instruct it to analyze all the sensor data the Enterprise picked up on the cube and its interior tricorder readings the away team took in Q Who episode and then request to try and develop weapon systems that would be effective against the Borg based on available data.
With superluminal processing, the computer would easily use latest in Federation scientific database to construct virtual models and run countless simulations and trials to test and see what might work and what wouldn't (even take into account Borg's adaptation into the mix).
By the time of 'Best of Both Worlds', the Enterprise should have had the computer develop and implement these new weapon systems automatically... and upgrade it multiple times over to the point where it wouldn't matter if the Borg had access to Picard or not (because as clever as he is, even he wouldn't be able to keep track of all the changes the computer would be making).
But I digress...
As stated, I suspect the Borg might be using the Federation to test their technological development and assimilate new valuable technologies that would help them in the Delta Quadrant... but save the AQ for another time.
Also, I liked the Destiny Novels, but they aren't canon.
And the Borg behavior in the novels simply doesn't track with how the Borg were initially conceived (without a leader/queen) and not really vengeful.
Though from an on-screen point of view... Voyager did strike a severe blow to the Borg in Endgame.
So, from on-screen sources, we might deduce that by destroying not just the TW hub, but also the primary Borg unicomplex in the Delta Quadrant, the Borg might have lost the information about assimilated future armor (the Queen did manage to only establish contact with that particular Borg sphere and gave it the information, however, that sphere was destroyed, so it's hypothetically possible that any future tech Voyager got, might actually be out of Borg hands).