Complex sentence: He can also, to some extent, see inside people's minds and inside fusions, as seen with his Gem friend Lapis Lazuli and The Cluster, an entity in Earth's core; as the series progressed, Steven's powers have gotten better.
Split into simpler sentences: He can also to some extent see inside people's minds and inside fusions, as seen with Lapis Lazuli and The Cluster. As the series progressed, Steven's powers have gotten better.