Grid-forming energy storage (GFM-ES) has become pivotal for renewable-powered islanded DC microgrids due to its voltage source behavior and power accommodation capability, with its dynamic characteristics directly governing transient voltage stability. Nevertheless, existing research lacks a clear theoretical framework for key parameters and dynamic interaction mechanisms affecting GFM-ES transient performance. Focusing on virtual synchronous machine (VSM)-based GFM-ES units, this paper establishes a hierarchical reduced-order control model grounded in multi-timescale control loop properties, systematically revealing transient characteristic evolution patterns. First, a layer-by-layer reduced-order control model is constructed according to distinct control loop timescales. Second, transient impact analysis is performed progressively using equivalent reduced-order models. Building upon this dynamic analysis, an innovative voltage support enhancement strategy incorporating dynamic power feedforward compensation is proposed. MATLAB/Simulink platform verification confirms that: the optimization boundary of power loop support capability is constrained by inner-loop control design; the proposed strategy effectively enhances GFM-ES voltage support capacity during transients.