Gaps between perimeter walls; potential under extrusion?
PETG filament is not entirely rigid and compresses slightly in the Ender 3's extruder gear and Bowden extruder setup. Tightening it will only make this effect greater. Being compressed at the point where mm of extruder advance is applied means less than the desired advance of at-nominal-diameter filament will take place. I find I need a flow of 104% to compensate for this.
When adjusting flow in Cura, make sure you get the main flow setting not the first-layer one (which is an additional factor on top of the main one and can be left at 100%), and that all the derived flow settings for each type of extrustion (walls, top/bottom, infill, etc.) all come out matching the value you set. When I first tried fixing this with flow, they didn't propagate right and I ended up testing changes that weren't actually doing anything.
With that said, you may have something else going on too. The underextrusion looks pretty severe, including in the top layers which you said looked okay. You should not see deep grooves between the lines like that. I suspect they're only bonded to the layer below, not to their neighbors. This could be a result of tightening the extruder pulley, or some other problem.