Hi @Rvarane,
as you probably already know, simulating FSI with thin structures and high velocities is complicated. Partitioned simulation has its limitations, but I am not sure if you have already reached that level.
The coupling and mapping settings of the perpendicular flap tutorial are already quite optimal, but you may need to further tune some numbers, such as the coupling time window size, the convergence measures, and more.
But before that, as you already identified, the main issues come from the meshes themselves, as well as from the mesh motion solver. You would definitely need some mesh refinement. You would also need a better mesh motion solver, such as RBFMeshMotionSolver: Mesh Solver OpenFOAM Error - #10 by Makis
Don’t forget to apply similar changes to the solid case, next to the changes you apply to the OpenFOAM case.