Understanding the termination dynamics of runaway electrons (REs) is crucial for predicting and mitigating their potentially damaging effects in advanced fusion devices such as SPARC. This work focuses on modelling RE termination dynamics using M3D-C1 and Geant4, with the aim of developing a workflow to assess RE-induced damage to plasma-facing components (PFCs). As part of this early-stage effort, we present preliminary 3D results on RE beam termination for SPARC, building on the 2D RE generation study by Datta et al. Nuclear Fusion 66 (2026) 046013. In parallel, a benchmarking activity between Geant4 and FLUKA is carried out to evaluate their capability to provide volumetric energy density profiles for thermal modelling. This work is preceded by a dedicated validation campaign of the Geant4 setup. This work is supported in part by the U.S. DoE FIRE Collaborative MiRACL under contract numbers DE-AC02-09CH11466 and DE-AC05-00OR22725 and Commonwealth Fusion Systems. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a Department of Energy User Facility using NERSC award FES-ERCAP0035392