You cannot endlessly increase the retraction distance, doing so leads to different problems as you encountered. As a rule of thumb, the retraction distance should not exceed the length of your nozzle. Depending on the type of extruder, many printers use a value between 2 and 7 mm (e.g. the Ultimaker Cura retraction length is 6.5 mm at 25 mm/s, this is for a Bowden style extruder).
In fighting stringing (in which PETG is very subjected to happening) retraction length is only one of the parameters that you need to adjust to optimize the printing process. Other important parameters (besides retraction length) for stringing are:
- retraction speed
- travel speed
- minimum travel distance
- flow modifier
-> From experience with printing kilometers of 2.85 mm PETG: if PETG is over-extruded by little, it causes the excess of material to form "curtains" like in the left image.
-> added by comments: PETG does not like to be "combed" over; moving back over the already printed material without retracting will "pull" it back up (due to adhesion to the molten material in the orifice) as well as pulling more material out of the nozzle; this results in junk all over the place which can manifest as "stringing" even though it's different from what "stringing" usually is.
Furthermore, the type of extruder (direct or Bowden) is also important and other parameters that may reduce the stringing are:
- coasting (stop extruding before the end of the print line is reached using the residual pressure in the nozzle to extrude the last filament)
-> Please note that coasting is a last resort as the amount of material needed for the whole print is not what is being extruded, i.e. stopping earlier means that the overall amount of material is less than what should be put in...
- pre-loading/priming filament after a move
Usually it suffices to tune the first parameters. As you see in your results, the retraction of 5 mm is close to the result you would expect, increasing it, may lead to clogs and in your case leads to a sort of stringing. This latter result is probably due to the fact that the filament does not reach the print in time (you definitely see under-extruded cylinder columns), while leftover pressure after printing the column layer oozes some of the filament creating strings.
Just play with the retraction length in steps of 0.25 mm (around the 5 mm) and you may try to change the speed with 5 mm/s increments if necessary. Also experiment with the flow modifier.