Note this is WINDOWS software but will recover MAC drives.
I had great success about 10 years ago with cnw recovery. It's $20 for a 30 day licence. You can download the software free and do a trial recovery, it will list all the files it can see but does not actually recover anything unless you pay.
https://www.cnwrecovery.com/ it's a UK company.
https://www.cnwrecovery.com/html/macintosh.html
You will need a drive to recover the files to. The software does not repair disks. If you think about it this is a disaster waiting to happen and is never appropriate.
I may just have been lucky. I pretty much knew what the problem was and I had high hope of success. I had bumped the drive when it was running and I used free tools to determine that it was likely that the Master File Table (or was it the MBR, I forget, it was 10 years ago) was not working. I was very careful to avoid writing to the disk thereafter, it was an external disk and not built in to a computer.
It is VERY important that the disk is not written to, if it is a computer System Disk (boot disk) I would take it out and put it in an external enclosure, or maybe at a pinch boot the computer from a different source. CNW state that the software will not attempt to recover a live system disk. e.g Windows boot disk.
Since you state that people have been working on it extensively, unless they took steps to avoid writing to the disk it may be beyond amateur (like me) help.
CNW used to do recovery themselves too, but I get the impression that the proprietor has retired and now only sells the software.