I will admit that I prefer to assume that "incubated drones" (like Hugh) were simply assimilated drones that were Borgafied at a really early age -- kind of like Seven was -- and so had no memories or identity to return to. The Borg baby from "Q Who" wouldn't have any previous memories or life to build on if we assume that it had been assimilated instead of being always incubated.
But that just seems so inefficient. Why rely exclusively on assimilating babies when you can just harvest gametes from the drones you already have and whip up some test-tube drones? And the Borg are nothing if not efficient. (Well, in theory, anyway.)
While Seven was far more emotional that Hugh was about being removed from the Collective, a huge part of her character was that she couldn't return to being Annika Hansen again and had to find a way to find out who she was now.
I always felt there was an irony there. Hugh had no identity other than as a drone, so his nature was to along with the crowd around him, and so when he was surrounded by individuals aboard the Enterprise, he readily absorbed their individualist values. But Annika was assimilated as a frightened child, undergoing the trauma of having her individuality taken from her by force, so when she was liberated by Voyager's crew, she resisted having another change of identity imposed on her. So Hugh took readily to humanity because of his innate Borgness, while Seven clung to her Borg nature because of her innate humanity.
My rationale is that the overall canon assumption seems to be that all Borg were assimilated at one time or another
That was the overall assumption of Voyager's producers. I'm not convinced that TNG's producers shared that assumption. It wasn't the way I perceived it when I watched TNG in first run back in the day.
, so I don't see the need to add the "incubated drones" that were not exactly stated in canon (as far as I know)
We were shown incubators in "Q Who." It doesn't get any more clear than that. Sure, if you filter that episode through the assumptions added by episodes written years later, you can force it to fit the assumption that all drones are assimilated. But if you'd been watching back in first run and "Q Who" was the only Borg episode you'd ever seen, you would've taken the baby drone in the incubator as exactly what it appeared to be -- an infant created by the Borg to be a drone. That was overt in the original concept, but it got reinterpreted in the revised concept that developed later. That's the trick of studying history -- you have to take care not to impose later assumptions when trying to understand how people saw things at the time. (Though it helps to be old enough to have been there at the time...)
Here's one more thing to consider: In TNG, all Borg makeup was the same. Every drone was just a pale human with machiney bits stuck on. We never saw what we later saw on Voyager, namely drones from multiple different species. Again, the idea of assimilation was paid lip service in dialogue in BOBW and later TNG episodes, but Locutus aside, it was never really used until VGR.