Storyline Auto Advance Slides

In that type of situation, it's not a good idea to advance a slide automatically. Yes, you can track that each flag/marker has been clicked. But that doesn't tell you whether the user has finished reading the content. And there's no way to know if they want to review what's in any of the markers before they move on. 

Here's what I suggest:

Add a trigger that changes the state of the Next button (the built-in one or whatever custom button you're using) to Disabled or Hidden when the timeline starts on the slide with conditions that check whether any of the markers ≠ (does not equal) Visited.

• There would be a "≠ Visited" condition for each marker. The conditions should be connected with "or" (not "and") so that the trigger will run if any of the markers hasn't been viewed.
• Why "≠ Visited"? Because the built-in states are layered on the Normal state, using "= Normal" in a condition doesn't always work as intended. As the late great Walt Hamilton used to say, the built-in states are like us: they all think they're normal. 😆

Also add a trigger that changes the Next button to Normal when the state of all the markers = Selected.

That will ensure that the user can't advance until they view all the content, and it also keeps them in control of when they're ready to move on. 

FYI, here's more about controlling the Next button.