I agree with your assessment for now, but I am curious about the outlook six months to a year from today. Usyk may begin to decline soon, especially as he turns 40 next year.
Regarding Kabayel, while he is clearly a talented fighter, I don’t believe he is on the same level as Klitschko or Usyk. I would also rate him slightly below Povetkin, Chagaev, and Fury.
In my view, he is above Wilder. As for Joshua, it depends on how he finishes his career, but for now, I would rank Kabayel slightly below him as well.
It's too early in Kabayel's career for me to really assess what his level is and especially where he'll rank versus other accomplished fighters. He's just gotten to world level and i need to see him against better, more proven competition.
As a side note, Wilder was a scary beast all the way from his title win through the first Fury fight. That trilogy seems to have wrecked him. Skills pay the bills and Wilder was always a few skills short, so once he got older, took some serious punishment, and got his mental f'd up he hasn't been the same. I'd love to see him wreck Chisora, though, and go on one last run. Nothing against Del Boy, but when Wilder is making highlight reel KO's I'm a happy man.
It may be early. Agit's is 33 and will turn 34 this September; hopefully, he secures a major fight by then. Time is not on his side.
While he has beaten several fighters ranked in the top 12–30, in my opinion, he has yet to defeat a top-five opponent. You can see his full record here:
https://boxrec.com/en/box-pro/575332
Wilder was a heavily hyped puncher with a significant fan base, yet he did not defeat any top-tier opponents. In fact he lost his biggest fights to Fury, Zhang, and Parker.
If you look at the ages of his best wins and his opponents in general, his most notable victory was over Luis Ortiz. At the time, Ortiz was 38 years old and potentially even older than reported!
https://boxrec.com/en/box-pro/468841