Hi Charly,
To achieve smooth result, you should avoid using Corner type vertices on the sloped sections of the base spline "Shape008". In the modified version of your scene, I’ve changed these vertices to Smooth. If necessary, you can also adjust the Style > Curve Steps value to further refine the spline. Let me mention that, in some cases, it might be necessary to add a few extra loops to the source geometry. However, that doesn’t seem to be needed in this case. I hope this helps.
Best regards,