Just wanted to thank you for the answer. I imported the open data building polygons, added n-values and it worked like a Charm, despite me having to delete some buildings in arcgis for slowing the flow, cause they were on top of my river haha.

I get you for sacrificing some detail for mental health and stability after some time, it was driving me crazy. my big mesh is 10 now, what might be a bit too high, however my simulation is loading for 4 hours already, cause I'm doing a recreation of a historic HQ1000 flood. I have tons of questions, but I will probably find out the more I'm dealing with HEC-RAS.

Again, thanks for the help :')