Can you set a retention policy to move a whole custom content type (based on document set, with documents inside) to different drop off locations and leaving a shortcut behind?
Also, will a drop off library every face the 5K limit, and do the short cuts left behind in the original location still count as part of the 5K limit?
One of my favourite bloggers has a post titled How to set up retention and deletion policies for files and folders in SharePoint which I think would be worth your time to read.
To answer one of your questions "...leaving a shortcut behind": yes, this is possible, it is known as 'In-place Records Management', the final section in this MS post will help you somewhat. He is a screen-shot of how it looks in SP (note - my screenshot is of a Library-based Policy, rather than a Content Type-based Policy. If the menu looks a little different, that is why).
Regarding your question on Drop-Off Libraries, this post has some info on how they work. I've not used these or read all of the post, but I understand the principle. I've used Information Management Policies - I imagine that you want to be able to configure a Policy to transfer a document to different location (a Drop Off Library) once certain conditions have been met e.g. 3 years after Creation Date. It looks like that would be possible, as you see in the picture above.
