Two points: You need to judge his career when it's over. And more important to your question, You need to have a definition for ATG.
As of right now, Joshua does not have a single signature win. The only wins he has currently which are even worth talking about in a discussion about ATGs are his Klitschko and Povetkin wins; both were over the hill, the former was inactive for 2 years and in his 40s coming off a loss, the latter was never even a real champion (Regular belt doesn’t count). So no, if Joshua knocks out Usyk he doesn’t become an ATG off the strength of one sole actual good win. Beat Fury and/or Wilder and then we can begin to have that discussion, until then, no.
Probably not by itself, but depends on what Usyk goes on to do. I think he probably has to beat Fury as well to reach that status. A win over Usyk and two wins over Fury would go a long way. Preferably win a trilogy over Usyk as well. If he got the better of both Usyk and Fury that might give him an argument for top 10 hypothetically. Throw Joyce in there and a second win over Ruiz and maybe also over Whyte and the argument starts to look quite decent, but there's a lot fo great fighters vying for the top 10. Liston, Frazier, Tyson, Marciano, Holy to name some. So the competition is fierce.
If he beats Usyk, possibly in a ruber match too, takes out Fury and doesnt lose anymore you could argue top 10.
Excellent point. Is Ken Norton or Vitali an ATG? Because I would rank AJ above both of them with a Usyk win.