Disruptions represent a critical challenge to the safe and reliable operation of future fusion devices like ITER, as they impose severe thermal and mechanical loads on the tokamak structure and generate high-energy runaway electrons (REs). This study expands on the work of [Vallhagen {\em et al}, Nucl.\ Fusion {\bf 64} (2024)] and presents a significant upgrade of the DREAM disruption simulation framework to investigate RE dynamics in ITER disruptions mitigated by Shattered Pellet Injection (SPI). The updated simulations account for four new key physical effects. The scraping-off of REs during Vertical Displacement Events is incorporated via a reduced model which reproduces the RE avalanche gain of higher-fidelity simulations. The plasmoid drift effect, which affects the deposition location of the injected pellet material, is accounted for via an analytical model which has been validated against experimental data on DIII-D. Additionally, a model for suppressing unphysical thin-current channels during the current quench phase is implemented. The Compton seed generation model has been updated with photon spectra reflecting the new ITER tungsten first-wall design. A wide range of realistic disruption scenarios are explored, including cases with low Neon SPI, scans of radial RE transport under varying magnetic perturbations, and trace tritium concentration scans to assess the impact of tritium beta decay RE source. The analysis of these scenarios guides the design of ITER SPI strategies, supporting the development of effective disruption mitigation techniques for ITER and future fusion devices.