Can not speak on cura but if one works then I don't think your issue is cura. PETG is very particular about over all layer height. Starting height has to be perfect or you are in for trouble. Each layer change need to be correct as well. .03 too high and you can have bonding issues. Too close and your nozzle picks up much and knocks off parts.

In S3d if I have a small part that would be 3 perimeters and no infill, I use 1 perimeter an 100% infill. with one wall and 45 and -45 zig zag infill with 18-20% infill outline over lap you get a solid wall with much stronger bond the a bunch of single treads stacked next to each other. Not sure if you can do that in cura..

My guess from what I have read on this forum about Ender 3 is most of your problem is due to uneven bed height as it raises ..