How to show non-popular checkboxes on button click in angular?

In a form I've got a list of about 15 checkboxes, of which 3 are the most popular. So by default I only want to show the 3 popular choices. At the end of the 3 checkboxes there should be an icon to expand and collapse the rest of the checkboxes.

I'm building the website in Angular, so I guess I can simply use an ng-show for this. The thing is that I wonder whether that is the correct way to do this. It seems to me that this is such a common thing, that there should be some kind of standard way of doing this..?

And also what kind of icon do people normally use for this? For expand and collapse I would normally use the little grey triangle, but I feel it doesn't suit this specific situation well. I know I've seen this before in other forms, but I just can't find it anymore to take inspiration from.

All tips are welcome!