Since WHEN was the captain the "star" of a Star Trek series?

Kirk was supposed to be the star of TOS, nobody expected Nimoy to steal the show from him.

And TNG and VOY may have had ensemble casts, but Stewart and Mulgrew were the standouts on both series. Although in Mulgrew's case I admit she stood out not because she was strong, but because most of the other characters were weak. Seven was the only one who had any real impact, and she came along halfway in. Stewart was marvelous in his role, he nearly saved the series from extremely dull writing.

There were no standouts on DS9 or ENT, in my opinion. Hope the new show can do better.

The Captain of every Star Trek show/ship has in fact been the "star". They have ALWAYS been the "main character" of the show, from Kirk to Pecard to Cisco to Janeway to Archer. That doesn't mean that, as you say, other actors/characters haven't "stolen the show", or had episode that focus on them over the main character. That has been a thing almost as long as television has been a thing.

Saying the Captains of the previous shows WEREN'T the "star", is like saying that Jerry Seinfeld isn't the star of "Seinfeld". He 100% is. That doesn't mean that George and Kramer aren't funnier and more interesting than Jerry himself. They are. But they're still no the "star" of the show. I love Kirk, and he's my favorite captain, but yes, Spock and especially Bones are more interesting and entertaining. But the show wouldn't be the same without Kirk, and he is absolutely the "star".

The problem that a lot of longtime ST fans have with Discovery, is not really that they are focusing on an officer instead of the captain (Which in this case is actually Michelle Yeoh). The problem many including myself have with this show, is that it not only visually (especially the "Klingons") reeks of the god awful Abrams reboot films. But that it also reeks of shallow modern Hollywood trends, such as trying to manufacture "Diversity", and leaning heavily on SJW-style talking points and themes. The very fact that they're trying to make events in this show "be a commentary on the current American political climate", is both absurd and embarrassing, as a longtime fan. When Roddenberry created the show in the 60s, he did not create it to be an analog to the current world of the 60s. He purposefully set it in the 23rd century, far enough in the future that he felt it would be plausible and acceptable by most audiences to see humanity shown as FINALLY having gotten over its petty bullshit like racism, sexism, and pointless wars. So tying this show to contemporary times is pointless, and lame.