How to access the content of an sd card formatted as internal storage?
From what I now today, I think the unfortunate problem is: Even without disk encryption Android always encrypts the "internally/adopted storage mounted" sd card.

As such you need to decrypt that. The file system used "afterwards" (i.e. inside of the encrypted "container") is, AFAIK, just ext4 or so. Basically a simple file system you can access with Linux.