A virtual synchronous generator (VSG) provides inertia and damping support for inverter-based resources, thereby enhancing grid stability. However, frequency coupling within VSG control reduces the system phase margin and may trigger low-frequency oscillations, especially under low grid-impedance conditions where conventional models fail to predict such instability. To address this issue, this paper presents a comprehensive sequence impedance modeling framework for VSG that incorporates frequency coupling, dual-loop voltage-current control, and sampling delays. In this framework, virtual impedance and frequency-coupling effects are equivalently represented as impedance components. A systematic stability analysis quantifies the impact of these components on phase margin and dynamic performance. On this basis, a two-step fixed-parameter impedance reshaping strategy is proposed that determines virtual impedance directly from steady-state parameters via closed-form calculation, unlike conventional methods that require iterative tuning. The proposed control strategy is validated through real-time hardware-in-the-loop (HIL) simulations on an RT-LAB platform. Experimental results demonstrate that the method effectively improves phase margin, suppresses low-frequency oscillations, and ensures robust VSG operation under low grid-impedance conditions.