Thank you so much for that hint.To backup your offline installers just use LGOGDownloader, it's only for Linux but it's what you need to do the job. It can get even the extras, and you can launch it also just to keep updated the stuff you already got. Just have a look at the command-line parameters and you'll see how powerful it is.
All I have ever heard of is gogrepo(c).py which does not work for me as it tells me there is a cpatcha required.
I will try out LGOGDownloader.
Linux is no problem