Disk mount error: 49244; repair exits with code 8

Question

I have a fairly new LaCie d2 4TB hard drive (roughly one year old). Some days ago it wouldn't show up anymore and now I looked into the issue.

In disk utility the disk is unmounted. When hitting "mount" I get this error:

(com.apple.DiskManagement.disenter-error 49244.)

When trying to repair the disk I get the exit code 8 and

Invalid node structure (8, 0) Invalid B-tree node size (8, 0)

I also have Bootcamp installed and Windows can see the drive; it seems normal to me there. But it is formatted as Mac OS Extended so Windows can't do anything with it. Do you have any idea what I can do other than reformat?

I read the in a different post that you can try the repair process in the terminal or boot into windows and eject the drive properly (yet, Windows isn't even mounting a Mac OS Extended journaled drive, so how should I unmount it. Also a normal shutdown should do this job, too, and I have done that multiple times in both macOS and Windows). I have tried the terminal commands from the mentioned post, the only interesting part is, when using the sudo fsck_hfs -r -d /dev/disk5s2 command I get this error, where it also shows error 7.

** /dev/rdisk5s2
    Using cacheBlockSize=32K cacheTotalBlock=65536 cacheSize=2097152K.
   Executing fsck_hfs (version hfs-522.100.5).
** Checking Journaled HFS Plus volume.
   The volume name is X-d2
hfs_swap_BTNode: offsets 1 and 0 out of order (0x0000, 0x0000)
   Invalid node structure
(8, 0)
   Invalid B-tree node size
(8, 0)
** The volume X-d2 could not be verified completely.
    volume check failed with error 7 
    volume type is pure HFS+ 
    primary MDB is at block 0 0x00 
    alternate MDB is at block 0 0x00 
    primary VHB is at block 2 0x02 
    alternate VHB is at block 7813365342 0x1d1b67e5e 
    sector size = 512 0x200 
    VolumeObject flags = 0x07 
    total sectors for volume = 7813365344 0x1d1b67e60 
    total sectors for embedded volume = 0 0x00 
    CheckHFS returned -1317, fsmodified = 0

(The files are backed up, so it would be ok to reformat, but I would prefer being able to repair it)

I would be very happy for any answers. Thank you!