Hey, thanks for all your kind replies

The problem turned ou to be the SATA to MOLEX cables that were a bit too bented, and therefore couldn't power the risers !
I put them in a better position and the cards were finally detected.
Now, I flashed the BIOS of the cards (with Polaris and ATIflash), but the fans don't spin anymore (only a bit when I turn on the MB, but when W10 boots, they go off and get really warm). Prior the flash they were always spinning !
Another fun story : once I flashed them, the memory type changes on GPUz (from Hynix to Micron, and then from Micron to Samsung...) I didn't take a random BIOS on the net, I just clicked "One click timing patch" on Polaris, saved the new .rom file, put it in the ATIflash folder, runned CMD and flashed one card at a time... However, on MB the memory type is the right one. So I reinstalled GPUz, and the memory went back to normal (except the fans, that still do not spin)
Should I wait more, and hope that the fans finally decide to spin? I'm afraid that they never will, and that the card will destroy itself

This rig is really driving me crazy

Help and advice would be very appreciated

Firstly nothing preventing you from making fan curve in software or settings static fan speed in software/miner.
Secondly, unless you checked only card-by-card, gpuz isn't ordering card normal.
Thirdly - How did you save your current bios from cards? Did all of your cards have exact p/n and similar s/n? Did you mod each one, or only one and flash it to all cards?