Described herein are stackable semiconductor device packages and related stacked package assemblies and methods. In one embodiment, a manufacturing method includes: (1) providing a substrate including contact pads disposed adjacent to an upper surface of the substrate; (2) applying an electrically conductive material to form conductive bumps disposed adjacent to respective ones of the contact pads; (3) electrically connecting a semiconductor device to the upper surface of the substrate; (4) applying a molding material to form a molded structure covering the conductive bumps and the semiconductor device; (5) forming a set of cutting slits extending partially through the molded structure and the conductive bumps to form truncated conductive bumps; and (6) reflowing the truncated conductive bumps to form reflowed conductive bumps.