Please add more settings to the attachments gallery so that it looks more professional.
* Show image name > true/false
* remove title (or make it configurable)
* image size
Sample comparing AGOL Beta-Viewer and Experience Builder Beta with a try to create a interactive skimap
Concerning the usage of popups > I would prefer to configure the popup behaviour in the application using it > e.g. add support to the experience builder to change the popup configuration for the current app.