I think you would find the Buffer tool can do this for you. https://pro.arcgis.com/en/pro-app/tool-reference/analysis/buffer.htm The Buffer tool has an option to get the required buffer distance for each feature from an attribute field. You would first need to add a new field to your attribute table that will store the required buffer distance for each of your river orders (what you call the"strokeweig" attribute).
I dont know how familiar you are will GIS operations in Pro so I will just add that the process I would follow would be:
1. Add a new numeric field to your attribute table
2. Then use the Select by Attributes tool to select one of your unique river order/strokeweig fields
3. Then with tool still open, click on the Open Another Tool button in top-right corner of geoprocessing tool to open a new tool without loosing any changes set in the Select by Attributes tool.
4. Search for Calculate Field, supply your attribute table as input then provide your required distance in the expression field.
5. Once done, click on the Open Another Tool to go back to Select by Attribute tool to modify the expression to select a new river order, then go back to Calculate Field to provide new distance values, etc.
6. Then when you run your buffer just point it to the new distance filed.