I am trying to get my head around what I think is a fairly straight forward geoprocessing problem:

I have created a polygon contour layer. However the polygon features are stacked on top of each other rather than having individual donut like features. Now when I use opacity some areas are thicker than others, due to the stacking of features, and I can't get a nice result (screenshot attached).

Any ideas which QGIS function and settings to use to resolve this? I came across this approach but it seems rather complicated for what I think should be a straightforward process: Exploding overlapping to new non-overlapping polygons?

Thanks!!