The emergence of wireless systems with numerous terminals, such as Internet of Things (IoT) deployments, Smart Home Systems, and enterprise Wi-Fi networks, requires efficient channel access mechanisms. However, existing methods in IEEE 802.11ax standard (Wi-Fi 6) face performance issues under practical non-saturated traffic conditions as a result of a trade-off between scheduled and contention-based transmissions. In this paper, we study a hybrid channel access scheme which combines scheduled and random-access transmissions into a single Trigger Frame (TF). Some Resource Units (RUs) are used for scheduled access determined by Buffer Status Reports (BSRs), while the remaining RUs are allocated for random access to accommodate new terminal connections. Realistic traffic modeling is considered using Pareto traffic arrival for non-saturated conditions. The analytical model is validated via simulation to evaluate throughput, access latency, ability to provide random access, and energy efficiency. The analysis is performed for the cases of both static and dynamic scheduling mechanisms. Results demonstrate that our proposed hybrid mechanism outperforms the random-only scheme, achieving up to 3× higher throughput than random-only access in areas of dense stations; reduces access delay by over 40% compared to static scheduling; and increases energy efficiency when using dynamic scheduling.