Microelectronic devices, assemblies, and systems include a multichip composite device having one or more integrated circuit dies bonded to a base die, a conformal thermal heat spreading layer on the top and sidewalls of the integrated circuit dies, and an inorganic dielectric material on a portion of the conformal thermal heat spreading layer, laterally adjacent the integrated circuit dies, and over the base die. The conformal thermal heat spreading layer includes a high thermal conductivity material to provide a thermal pathway for the integrated circuit dies during operation.