How can I add valuable details (like insights, quotes, images, and links) to a map, without making the map cluttered and still be able to get a nice overview? 

The answer is: Use sub-lanes and place your details in the sub-cards in this lane. That way you can hide details in your maps when presenting a high-level journey, as well as drill down in the map when you need to get to the details. 

Sub-lanes and the cards in these lanes (called sub-cards) are connected to the regular lane and cards above. They can be used for additional information, links, documents, or images, and GIFs. And they can be hidden or shown, by a tap on the small triangle in the card lane above them.

Note: that a sub-lane can only exist if there is a regular lane above to connect them to. And you can only add a sub-card to a sub-lane if there is a regular card in the regular lane above. You can add as many sub-lanes (and accompanying sub-cards) to a regular lane as you like.

Click on the small triangle on each card to fold out the sub-lane and sub-cards underneath. To hide sub-lanes, click on the triangle again. You can also hover over the lane headers on the left side of the map, and then click on the text “show sub-lanes”