And the approach mins have to be corrected, that is, those AFTER the FAF.

From Airbus “ Gettin grip with cold weather ops “ :

“ On approach, at least the following published altitudes must be increased in low OAT conditions:
• MSA,
• FAF altitude,
• Step-down altitude(s) and MDA(H) during a non-precision approach,
• OM altitude during an ILS approach,
• Waypoint crossing altitudes during a GPS approach flown with vertical navigation.
As it is not allowed to modify the altitude constraints of a non-precision approach, a minimum OAT to fly the approach with the «FINAL APPR» FMGC mode must be established.
For OAT lower than this minimum, selected vertical navigation must be used.”

If you choose to stay in an uncorrected altitude of 3000ft, than you have to start your descent later and apply the cold temperature correction on your FPA.
So -20 gives you 3,5*0,04= 14%. This means if you start your descent in DUS at 7.6 NM with an FPA of -3,4 you should be fine.

We only have to correct altitudes below the corrected MSA. So in this case all below 3192ft

Does your company prevent you flying VNAV down to the LNAV minima? If it does then if you are at a non-temperature corrected platform, you will indeed have a shallower or later decent to the first temperature corrected checkpoint. If it is allowed then the aircraft will start the decent slightly later to maintain a constant angle. (Speaking from Boeing perspective anyway...)