It seams it might be intresting to consider the differance betrwwn a static GIS based model like g.wathershead with mor dynamic modells where percipitation can be varied is time and space. It looks like synxflow could be an intresting starting point. Also landlab has potential. Although landbab is more focused on landscape forming processes ther is a tutorial including a video on using landlab for flooding modelling available at https://hatarilabs.com/ih-en/flood-simulation-from-direct-rainfall-with-python-and-landlab-tutorial Finaly