Hi,
From what I gather, blueprints of classes inheriting from AActor draw thumbnails from primitive components, if they’re present in the actor.
One solution to your problem is to add an appropriate looking primitive component to the parent class and it should render in the content browser.
The code in the engine that searches for thumbnail renderable components(in BlueprintThumbnailRenderer.cpp) already loops through parent classes as well, so it should work.
BR,
Thraden