With the increasing incorporation of renewable energy sources into modern power systems, the resulting uncertainties present significant challenges, especially in controlling frequency fluctuations within isolated microgrids. To address these challenges, this research introduces an innovative approach called ONPIVIC, which combines an optimal Nonlinear Proportional-Integral (ONPI) controller within the virtual inertia (VI) control loop. This novel Virtual Inertia Control (VIC) method is designed to enhance frequency control in low-inertia microgrids with varying levels of renewable energy sources. The proposed strategy involves an optimal design based on a nonlinear PI controller and a virtual inertia control loop using the Arithmetic Optimization Algorithm (AOA). This approach enables the VIC unit to regulate the inverter-based Energy Storage System (ESS) while simulating different inertia levels and responding effectively to fluctuations in generation and load. To compare the performance of the proposed controller, two alternative control methods, namely the Optimal Virtual Inertia Controller (OVIC) and the Optimal PI-based Virtual Inertia Controller (PIVIC), are also considered. The effectiveness of the proposed controller is evaluated through five different scenarios that account for sudden changes in power demand and generation in isolated mode. The results demonstrate that the proposed ONPIVIC controller enhances frequency stability and control efficiency in microgrids incorporating renewable energy sources, facilitating a smoother transition to cleaner energy generation.