I've seen it said that TNG was centered around Picard, Riker, and Data like TOS was centered around Kirk, Spock, and McCoy. But I've always thought of TNG as an ensemble cast. Thoughts?

There's plenty of Troi, Beverley and Geordie episodes in the mix, and worf episodes are typically excellent if few.

But I think the big three you mention dominate the story lines as they're running things whenever a story isn't about a particular crew member

I feel like every Worf episode was them trying to remind you that he's supposed to be a bad ass, even though he gets his ass handed to him every other episode to show how much more bad ass the enemy of the week is.
- Worf may be a badass, but there are many that are just badder than he is. But as the Jem Hadar guy said "I cannot defeat this man, I can only kill him".
Are Worf episodes even few? Seems like he's just under Picard and Data in terms of focus. He gets to be the focus of a whole season ending two-parter and gets multiple arcs throughout the series. More then Troi, Geordie, and Crusher for sure.
- There's heart of Glory, sins of the father, redemption & birthright for worf and his heritage I think. And parallels which is just about worf.
  
  It's not many unless I'm missing some.
The funny thing about the Troi episodes are that the best ones with her in them are when she isn't being a counselor.

No TNG was definitely a full ensemble cast. I think people say that because Data is an obvious analog of Spock. But if this analogy is to hold, who is McCoy? Riker? Crusher? It doesn't make sense.

I think they just held onto the TOS traditions of strong captain as the lead with a Spock-like character and diverse remaining cast. But TNG had more space to work with the non-lead actors than TOS did partially because it had TOS's world building as a foundation.

Keep in mind TNG ran for a more then twice as long as the original series. Gives it a lot more time to flesh out its cast of characters. Originally Picard, Riker, and Crusher were envisioned as the main trio, but all of early tng's production issues and Gates Mcfadden leaving for the 2nd season threw a wrench in that vision. Starting with season 3 the show becomes more of an ensemble. You start to get the formula of specific character-centric episodes that will stick with the show for the remainder of its run. Honestly it feels like near the end of its run TNG was more the Picard, Data, and Worf show. Riker gets hardly any episodes that focus on him in the last few seasons.

I think Gates McFadden left because she thought she wasn't getting enough lines or something. Funny because I think she got even less later.

I agree it started out with more Riker and he didn't get much toward the end. Just the "what the hell are we going to do now?" line
- She didn't leave. She was fired. Watch recent interviews with her.

All the post-TOS shows expanded and deepened main cast, leading to episodes being able to be centered on various characters. TOS had a smaller and more focused cast. It was a rarity to even have a Spock or McCoy centered episode, and I can’t think of an episode where at least one of the main trio wasn’t the most prominent character.

I think it’s just a difference in writing styles of the different times between the 1960s and the 1980s. Even look at something like STIV: The Voyage Home. Same characters as TOS, but written in the 1980s and a lot more comfortable giving various non-trio characters their own individual adventures.

Another important factor is TNG's reliance (sometimes over-reliance) on A/B plots. The B plots were often an outlet for the ensemble characters to come out and play.

I think you can even argue that in each case there are two main characters plus a third wheel (Riker/McCoy).

But McCoy never faded from view, whereas Riker almost became a background character during the second half of TNG. They should have written Riker out of the series after Best of Both Worlds; after the character stuck around, the writers seemed unable to figure out what to do with him.

I think they did a better job than TOS did writing stories for the other crew members, but the last few TNG movies are basically the Picard and Data show

Yeah, and the problem with the Picard and Data show is that those characters don't really have a relationship, at least not an emotional connection like Kirk/Spock/McCoy. They basically just had a professional relationship, which was fine for the series where there's a problem to be solved every episode, and it's not necessary to have fleshed-out character arcs. But a movie narrative needs to make the audience care about the main characters and their interactions with each other, all within a very short period of time. Picard/Data simply could not provide that emotional core.