Thank you Luuk!!
I do wanna use the picker to pick content (document or media) in the basic way. But good to know that the other way exists.
Future?
Im still searching for the official way. I cant really find any solid docs on any of the suggestions. And @nielslyngsoe even suggests there is a different method in v16 (Opening a Content Picker modal from a Custom Property Editor - #5 by sm-rob)
Update:
I found
But .. this is the kind of docs that I cant read nor understand.