It was clearer in earlier edits of your post, I think …
So, your device is star2lte - Samsung Galaxy S9+, not a Fairphone 3 with which this topic started out.
Since the mkdir -p /mnt/userdata command doesn’t complain, /mnt/userdata should exist (which your later edits confirm). This would mean /dev/block/bootdevice/by-name/userdata doesn’t exist. The structure may be different on an S9+.
It says in https://gitlab.e.foundation/e/backlog/-/issues/6769 …
“We now have a recovery plan for that situation. Do not hesitate to reach out to us in case you face this problem again.”
So perhaps you could reach out by contacting helpdesk@e.email.
(They didn’t say whether the recovery plan was specific to the Fairphone 3 or more universal.)
Else … @tcecyk: You had some ideas in How to mount encrypted /data partition in recovery mode? … Where to start here 
?