What's the point with that quite lackluster story in Endwalker?

[Absimiliard]

So what your interpreting is that Golbez and Zemus is a discount Arthas with Nerzhul.

Ok good to know

Eh, not so much. Arthas was the one calling the shots inside the Lich King. He consumed Ner'zhul at the end of Arthas: Rise of the Lich King. This consumption was not complete, however; part of Ner'zhul remained, struggling to regain control. Unfortunately, Arthas used Ner'zhul's guilt at playing a major role in the downfall of his own race to send him into a spiral of despair that not only made it impossible for him to ever claw his way back up, it resulted in his remnants' near-total dissolution over time. Only faint echoes and some memories remained by the time of Arthas' defeat.

Then along came Shadowlands to ruin a bunch of crap by introducing the Jailer, who apparently tried (and failed) to control the Lich Kings via the Helm of Domination. Ol' Ner'zhul's soul wound up in the Maw at some point, enslaved again.

With Zemus and Golbez, it's more of just the former spending years to wear the latter down until he could be made to serve, but never quite managing to completely break him.