I backed Mike Shea's last "campaign" type book, and it was a great setting and collection of adventures. It was designed for levels 1 through 5 (similar to Lost Mines of Phandelver), but then at the back he included an appendix with ideas for, IIRC, a level 5-20 campaign. It wasn't super detailed, but it was enough to give DMs an idea what to do if their players really liked the setting and wanted to stay down there.
You know how you can read a campaign book and be like, "Why is this so confusing and hard to understand? Don't they care how hard the campaign is to run?!"
Well, Mike does care -- that's his whole shtick, so I'm definitely going to back this.
I talk only as much as is necessary to paint the scene and hurry to prompt player action.
There's an old bit of advice I read somewhere that the sooner you ask players, "What do you do?" the smoother your game is running.
Those really old AD&D modules with 3/4th the page taken up by boxed text? People tend to zone them out. WotC did studies on this and figured attention starts to drift after 2-3 sentences.
But it goes beyond boxed text. Any time the GM is sitting there talking, be it narration, exposition, or -- worst case scenario -- two NPCs having a conversation, that's time the players have to sit there trapped in an unskippable videogame cutscene.