I managed to get the Polygon to centerline Tool.
Running it takes quite much time as I need to create the centerline for 800+km of rivers.
After having created the centerline for a test I buffered it with 501cm because i need rivers less wide than 10m.
I then selected the buffered parts with "select by location". Selection method "select features from", Target layer "centerline Buffer", source layer "parts adjacent to the river", spatial selection method for target layer features "intersect the source layer feature". Search distance "off" (I possibly could also define search distance to 501cm?!) Then I create a new layer from selected. I then use the intersect Tool to combine the rivers with the buffered Centerline so that the Polygons from the center line do not exceed the river.
It probably isn´t the most efficient way but it is precise enough for what i need.
I tried to increase the speed of Polygon to centerline with dissolve but it still takes VERY long.