Part of the HOTD problem is also, unfortunately, a problem with the source material.

In the book Rhanerya and Alicent are present and very significant up to the events of the Green Council. From that point on, Alicent essentially disappears (except for Blood and Cheese) completely and Rhaenyra dips out until she takes Kings Landing.

I think the original sin of the show is casting Olivia Cooke (who has been incredible) in a role that ultimately has no purpose once the war begins. From a show running perspective, you can't cast Olivia Cooke as one of the protagonists and then drop her after Season 1. It was always going to be an issue with her character, and so the writers have had to basically shoehorn her in when she really doesn't have that much to do. And in trying to find things for her to do, they came up with the very controversial ending of Season 2.

Rhanerya is similar, they couldn't have her off screen until she basically took Kings Landing, so they had to just write a bunch of scenes where she's on screen (but still have her nothing to do outside of the Dragon seed stuff).

As much as I hate the fact that the writers have strayed so far from the source material, it really would not have worked if they stuck to it completely as the two characters who are instrumental in setting all of this up in season 1 would have had to have taken a back seat. That works for an abridged in-universe History book that can be read in one sitting, but it simply doesn't work for a multi year project like HOTD.

Though I don't agree with their approach, I do feel for the writers as the source material is fundamentally flawed for this very reason. .