Repair options of a (really) Broken HFS+ "Invalid extent entry" volume?

I have an un-mountable external 2TB harddisk, formattet HFS+, one "usable" partition, containing a set of my backups. It won't show in Finder, it won't repair in disk utility, it won't repair on the commandline:

$ diskutil verifyVolume /dev/disk2s2
Started file system verification on disk2s2 zuhauseBackup
Verifying file system
Volume is already unmounted
Performing fsck_hfs -fn -x /dev/rdisk2s2
Journal needs to be replayed but volume is read-only
Checking Journaled HFS Plus volume
Invalid extent entry
The volume   could not be verified completely
File system check exit code is 8
Restoring the original state found as unmounted
Error: -69845: File system verify or repair failed
Underlying error: 8

$ diskutil list /dev/disk2
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS NameNotShown            2.0 TB     disk2s2
$

(The above NameNotShown is, in reality, shown correctly).

Linux hfsplus-fsck also give the same error as the macOS tools (it appears to be exactly the same tool):

$ sudo fsck_hfs /dev/rdisk2s2
** /dev/rdisk2s2
   Executing fsck_hfs (version hfs-522.100.5).
** Checking Journaled HFS Plus volume.
   Invalid extent entry
(4, 0)
** The volume   could not be verified completely.
$

Mounting it to a Linux PC I can see some files in the Backup-Directories, yet most "important" directories only show up as files of size 0.

The above Invalid extent entry seems to be the culprit. Any ideas how to get around this and possibly fix my volume, hopefully at least recover the files?

Solution 1:

Though it still cannot handle APFS, the single best 'fixit' app for HFS+ has always been DiskWarrior. idk whether they do a free trial.

Failing that, you're into 'file scavenger' apps, which are going to be poor on a drive full of hardlinks.