[abenedik]

Maybe I am missing something, but from how I understand the problem the following may solve your problem:

When you begin building your scene create a new List<InstanceData> (try to predict the number of instances and call the constructor of the List with that number).

Then during building of your scene, when you see that you need a new instance, just add a new InstanceData to the list.

At the end of building your scene, create a new InstancedModelGroupVisual3D and convert the List<InstanceData> to array and set it as InstancesData.