How to allow read/write access in a specific folder on the SD Card with Android 6.0.1?
Does the SD card have to be removable? If not, Android 6 (Marshmallow) allows write access to a suitably formatted SD card (HowToGeek article).  It effectively makes the card an extention to internal storage, so use a fast one (PCAdvisor article). Also be aware that reformatting makes the card unreadable by any other device.

I had a similar problem with several apps not having write access to my extension card.  It was solved by:


Resetting all apps moved to the SD card back to internal storage (not needed on 6.0.1+).
Backing up all my own files to my laptop using USB link.
On Settings->Storage->SD Card choose to Format as internal storage.
Restoring my files.


I expect you will then be able to link to your AnkiDroid folder.  But you may not need to.  In theory, Android will do the memory management for you.  If it does, please let us know.