They would have to call cubes back from where they are pushing territory on the other side of their territory.

The Borg were not just fighting one species when they came to get the Federation, they were expanding outwards on all sides. So they committed the lowest level of resources they believed were necessary, and because the Queen was an arrogant fool, that was just one cube.

For First Contact, you can argue that having been thus far unable to assimilate the Federation they are unaware of the speed of human advancement. In the Star Trek Universe it has been implied that humans are EXCEPTIONALLY inventive especially when faced with a problem, and that the Federation is even FASTER than humanity alone because of the additional viewpoints added to human inventiveness. Basically, the Human Problem of Fantasy Games where the humans are an average, all-around boring species while Elves and Dwarves and others all have specialties? That’s not applicable to Star Trek Universe, where humans are especially well-suited to be engineers, and highly valued for their social abilities which foster teamwork. The presence of humans in the Federation is one of the ingredients that makes the Federation uniquely effective at technological advancement. Not only is the Federation large and powerful, it advances more quickly than the species that the Borg have assimilated, and has advanced to a level that the Borg never allow other species to advance to, AND it advances the way the Borg do by peacefully trading and adding technologies when it admits new member species.

The Queen never dealt with a society like the Federation before, and she didn’t expect them to advance very far beyond their capabilities at Wolf 359. She figured her cube was better, and that should be good enough and if by some weirdness it wasn’t she would destroy the Federation by going back in time and destroying its weirdest, least predictable species: humanity.

Not if Wolf 359 motivated Starfleet to vastly improve their weaponry on their ships specifically to engage the Borg.

For example, if frequency changes on the phasers and shields weren’t standard prior to Wolf 359 but were after, then a single Federation ship has a much better chance against the Borg because they can adjust to the Borg adaptations. That’s without an increase in actual damage capability, which was almost certainly on the menu for any ships built after Wolf 359.

Voyager was the first of her class, and she launched shortly before the Sovereign class and the Defiant class which were ships designed SPECIFICALLY to take out Borg. Voyager was designed and built while they were building their escorts and tanks to hold up against the Borg, so it would have had a number of defense advances that had been leading up to that level. And nearly every species Voyager encountered considered it a tank and a warship rather than a science ship, and most were in awe of the level of firepower it held. (They’d have probably turned tail and run if they saw the Enterprise E.)

The Federation, like they did between Discovery and TOS because of their war with the Klingons, tanked up their ships and loaded them with new firepower after getting their asses kicked and getting reminded that exploration can be a dangerous endeavor.

Q’s involvement is from the computer game Star Trek: Borg. It’s a fun, if mostly linear, Choose Your Own Adventure type game shot on the Voyager set.

The FMV PC Game Star Trek BORG. Here’s a video edited down to a movie format: https://www.youtube.com/watch?v=hi2HO5e_oeM