Messy top layer with PETG filament?
Five top layers should normally be more than enough to create a seamless top layer.

Indeed, PETG prints a little differently than PLA. It requires a higher hot end temperature, less part cooling (to improve sticking to previous layers), a higher build plate temperature and usually care in choosing the right initial layer height. Once the printer  needs to create the top surface layers, too much temperature, too less cooling and too low of an infill percentage can cause the top bridging over the infill to fail. Your settings seem to be fine except for the low percentage of infill, 15% is very low.

Other possible causes could be under-extrusion and too fast printing, but in this case the low infill percentage is probably the main reason. To get a better top layer you could first try to increase the infill percentage. If you go to 20 or 30 % infill, you would only marginally "waste" filament. More filament and time is wasted when whole prints fail as of a bad top layer.