QGIS - Buffer distance as portion of polygon variable width

I think you could work on a raster based approach to solving your problem. What you ideally would like to have is some sort of inversed Euclidian Distance calculation (from the polygon's exterior towards its inside.

You could attempt this as followed, I haven't tested but can get you started:

• use Polygon to Lines tool to convert the river to a line segment.
• Use Euclidian distance tool to calculate the distance towards the outside.
• Use the river polygon shape as the mask or extent of the Euclidian Distance calculation.
• If needed, convert the Euclidian Distance Raster back to a polygon using the polygonize tool, after resampling and extracting desired distance from the internal river line.

There are some articles that try to attempt this that might be of use as well:

• Find out if overlaying polyline matches polygon centerline in qgis
• Calculate Polygon Centerline with a LeastCostPath approach