Die-casting molds undergo wear, erosion, and repeated thermal loading, which progressively degrade their surfaces and reduce their service life. Compared with conventional gas nitriding, sulfonitriding can improve wear resistance; however, its effects on other reliability-related properties, particularly erosion and thermal-shock fatigue, and the relationships between these properties and the compound-layer characteristics remain insufficiently understood. In this study, duplex surface treatments combining sulfonitriding with three microshot peening conditions, resulting in surface-roughness values of Rz 14, 18, and 22 μm, were applied to SKD61-based (FDAC) die-casting mold steel. The duplex-treated layers were evaluated based on their microstructure, hardness, wear resistance, erosion resistance, and thermal-shock-fatigue resistance, followed by a nondestructive assessment using Barkhausen noise. Sulfonitriding produced FeS-containing compound layers and reduced the wear of the counterbody and the tribo-pair compared with the results for gas nitriding, which is consistent with the solid-lubrication effect of FeS. The erosion resistance increased with the compound-layer thickness. The thermal-shock-fatigue tests indicated a preliminary trade-off between crack initiation and crack propagation as a function of the compound-layer thickness. These results demonstrate that sulfonitriding modifies the surface performance of SKD61 and that the compound-layer thickness is a key parameter governing mold reliability. Moreover, Barkhausen-noise analysis offers a nondestructive assessment of the compound-layer thickness after treatment.