The fifth-generation vehicle-to-everything (5G-V2X) communication technology is under development by the Third Generation Partnership Project (3GPP) mainly to support direct communications among vehicles over the sidelink interface. Both periodic cooperative awareness messages (CAMs) and sporadic alert messages, such as Decentralized Environmental Messages (DENMs), are transmitted in broadcast over the sidelink to support basic safety applications and more advanced cooperative driving use cases. The two classes of messages may suffer from poor reliability either when considered individually or when competing among each other for the limited radio resources, which are accessed without a centralized coordination. In this paper, we analyze the DENM performance over the 5G-V2X sidelink under the autonomous resource allocation scheme, also known as Mode 4 in 3GPP Release 14 and as Mode 2 in the next Releases. Message repetitions and time-frequency diversity have been exploited to improve the DENM performance by also using Maximum Ratio Combining (MRC) at the receiver. Achieved simulation results show improvements in terms of DENM reliability, with a negligible impact on periodic messages.