I'm somewhat puzzled which settings I need to look at in PrusaSlicer to solve those thin slits between perimeters. I've dialed in the flowrate optimally for dimensional correctness and 245°C (PETG) works really well for both flow and overhangs.

At first I thought of extrusion width, but that also increases the distance between each line. Raising the flowrate closes them, but also makes the printed parts grow in each dimension outside if the intended size.

Where am I missing something? Which settings do I need to adjust to not screw up everything else?

EDIT: It's not just corners!