Relativistic runaway electrons (RE) with energies in the MeV range can be generated during tokamak disruptions. A RE beam carrying multiple MAs of current poses an intolerable risk to plasma facing components. To explore RE physics and alternative mitigation concepts, RE expulsion from the plasma core using ECRH has recently been investigated in TCV [1]. In this contribution we report on the study of RE transport by approximating RE diffusion coefficients using Bayesian optimization with DREAM fluid simulations. Experimental measurements of the spatiotemporal evolution of the RE momentum distribution are necessary to improve physics understanding and validate numerical models. We have applied tomography techniques [2] on select ASDEX Upgrade (AUG) RE experiments for this analysis. This work is also part of the theoretical work conducted in preparation of the installation of a MANTIS system on AUG. --- [1] J. Decker et al. Nuclear Fusion (2024) <https://dx.doi.org/10.1088/1741-4326/ad6c61> [2] T.A. Wijkamp et al. Nuclear Fusion (2021) <https://dx.doi.org/10.1088/1741-4326/abe8af>