You are correct. I agree, and what you say about these three characters is something I've also felt must be true. In my earlier versions of this theory, I've often brought up recurring characters and elements, such as Khadgar, Outland, Jaina, the Lich King, Thrall, etc., with Gul'dan, Sylvanas, and Iridikron being these "key" figures that unlock the next part of the story.
You could say Garrosh was the first, who gave us WoD after MoP. I think that's when this type of development started. Tom Chilton said as much in interviews at the time, IIRC.
For anyone in doubt, it's also important to look at it this way:
Each story continues until it culminates.
So it's not weird that N'Zoth is in BfA, even if the Void has its final showdown in 11.0. Because the Void storyline is last (after Disorder and Death), it had more time in which to keep developing. It's a longer story.
You could argue it needed all this time, too. The Disorder story started all the way back in the RTS games, as did the one about Death. The last of the three (Void) only really started in WoW. Maybe you could argue it had some traces back in Warcraft 2 and 3 with Deathwing, Thrall, and various "Ancient One" units, but it was nowhere near as defined a concept as the Burning Legion or the Scourge.
In WoW, the "Death" story continued from Vanilla all the way until Shadowlands. Then it pretty much stopped.
The "Disorder" (Burning Legion) story stopped even earlier. It went from Vanilla to Legion.
The "Void" (Old Gods) trilogy is still ongoing. It will have had 10 expansions' worth of exposure before it finally ends, which it pretty cool.