Aqueous rechargeable zinc-ion batteries (ZIBs) are a promising addition to the energy storage landscape, particularly supporting the decarbonization of the power sector (i.e., deployment of wind and solar) as a low-cost, high-safety alternative to lithium-ion batteries. Unfortunately, wide-scale ZIB utilization is hindered in part by limited energy storage capacity and operational durability of state-of-the-art Mn oxide cathode materials. As such, we take a combined approach to investigate the impact of simultaneous inclusion of alkali metal (Li, Na, K, Rb, and Cs) and Ni additives on the structural and electrochemical properties of Mn oxide-based cathode materials for ZIBs. We used a facile, scalable synthesis approach to prepare 15 unique Mn oxide-based cathode materials and identified several materials capable of delivering a practical rate (i.e., C/10) discharge capacity of >150 mAh g-1 and a fast charge (i.e., 1C) capacity retention of >90% after 200 cycles. Detailed characterization of the prepared materials revealed the use of alkali metal additives during synthesis impacted both phase structure and electrochemical performance. Modifying the phase structure of Mn oxides resulted in an elevated Zn2+ diffusion coefficient for K-containing materials and a subsequent increase in deliverable capacity. Furthermore, the incorporation of Ni in the structure was shown to have no meaningful contribution to improved capacity within the aqueous electrolyte stability window, although a slight uptick in capacity retention was observed for all prepared materials as Ni content was increased. Considering the narrow range of reported cathodes for ZIBs, this work provides insight on the important interplay between structure, composition, and improved performance of Mn oxide-based cathodes, helping accelerate future material development efforts necessary for ZIB commercialization.