Vehicle-to-everything (V2X) connectivity is deemed crucial to enable upcoming cooperative automated driving applications. The Cellular-V2X (C-V2X) technology supports direct interactions among vehicles over the sidelink interface. According to the autonomous mode, vehicles select sidelink radio resources for their message transmissions in a completely distributed manner, without the support of the base station. The majority of works focused so far on periodic message transmissions, for which sidelink resources can be semi-persistently reserved after a channel sensing procedure. In this paper, we take a step forward by considering both periodic and high-priority aperiodic messages, such as sporadic decentralized environmental notification messages (DENMs), competing for the same resources. The impact of some new sidelink features devised in 5G-V2X (Release 16) for the autonomous mode, such as the new radio (NR) flexible numerology, the short-term sensing, the pre-emption mechanism, are preliminarily investigated in this paper. They are coupled with a newly proposed resource reservation scheme for the repetitions of DENMs. Results showcase the improvements of our proposal in terms of higher DENM reliability achieved with little-to-none additional signaling, with a negligible impact on periodically transmitted messages.