In Modeler 8.3, it's not possible to create N² Analysis matrices.
I propose to enable this, by going further with the Custom Simple Matrix.
Both rows and columns should be the same: they are the action nodes of a specific activity. The intersection is if 2 action nodes are interacting in the activity diagram of the activity i.e. if one action node sends/receives an object flow to/from the other action node.
Ideally, such a function would also include a way of sorting/grouping the activities easily based on the number of interactions, in order to ease the identification of high-interaction areas.