Hello Wido
This is the typical scenario that requires a data recovery specialist.
Though search my posts where I explain that the biggest error that people do when They do DIY data recovery is to skip to diagnose the single drives SMART values with utilities like CrystalDiskInfo and next big error is to skip to clone/image them.
If they care their data, they should bear in mind that the unique copy of their data out there are the ones present in the RAID original drives. If one is failed, also the others (especially if they have the same age) may fail or even die during the data recovery operations.
In the mean time if SMART are bad or in warning, the drives must be cloned with specific hardware devices that avoid to stress the disk when bad sectors are encountered: nobody knows how many bad sectors are present and at the end they could be thousands. Cloning with PC and software will hang many time on each of them, killing the drive.
Last: we do receive sets of disks made unrecoverable by crazy or self-killing actions made by the users while playing with the NAS web interface. The worst ones that happen are conversions to RAID 0 or 10, RAID full initializations and stale drives forced to return online (destroying the entire parity structure)
Said that, this looks the typical scenario that explains why they do exist raid recovery specialists
If you need help, you can write me a PM and eventually forward us the drives. We do provide a variety of flat rates that are linked to timing only, so if you are not in a hurry, you can get really a cheap fee. Also we can do remote pre-diagnosis for free.
Regards
Cor.