It sounds like you're already on the right track with adjusting the extrusion multiplier and experimenting with different extrusion widths. Overextrusion on top layers can be caused by a few different factors. Here are a couple of suggestions to further troubleshoot the issue:
Calibration: Make sure your extruder steps/mm are properly calibrated. You can do this by measuring a known length of filament that enters the extruder, then comparing it to the actual extruded length. Adjust the steps/mm in your printer's firmware if necessary.
Z-axis alignment: Check if your Z-axis is properly aligned. If it is slightly off, it can cause inconsistencies in layer heights, leading to overextrusion on the top layers.
Cooling: Insufficient cooling can also cause overextrusion on top layers. Ensure that your printer's cooling fan(s) are functioning properly and that you have adequate airflow around the printed part.
Retraction settings: Improper retraction settings can contribute ...
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