Upvote:1
Technically, you are correct that carrying objects out and back in is customs-free, without value limits.
Practically, probably nobody will ask ever, if you just walk through the green exit.
If you ever get stopped, you should be able to simply declare that all these items were bought before the trip, and simple carried in and out; and for smaller value items with obvious usage, you will be good - after all, nobody claims you might have bought all that dirty laundry in your bag on this trip (which you actually could have done...).
Typically, they care about high value items that either look brand-new, are in original packaging, or where you seem to have an uncommon amount of (for example, 4+ iPhones). For such cases, it is a simple solution to photograph the item with its bill when you buy it, and keep the picture on your phone.
Otherwise, I think you are overly worrying. I travel a dozen times a year into Germany, and mostly have two laptops (company and personal), an iPad, two iPhones (again, company and personal), and two cameras with several lenses with me, easily 20+k€ overall, and mostly without any scratches or dents. Nobody ever cared the least; that seems to be considered 'within the non-conspicuous limits'.
Having a single iPhone inside the original box, however, it would be quite hard to convince customs it was not bought on the trip.
I understand that this is not a real solution, as the small chance of annoying trouble is still there; but realistically, it will probably never be an issue. The chance of something getting stolen during your trip is probably much higher.