Hi everyone !
Resolved!
After repeating the error by removing the data disks and replay the procedure with only 1 test disk:
You are right, Bi0sh0ck!
I was foolishly wrong to switch directly from version 5.x directly to 6.2.2 update 3.
Here is the correct procedure: I reset the DSM partition on ALL the disks on the raid with wonderful Minitool Partition Wizard *free*
https://www.partitionwizard.com/free-partition-manager.html
like this excellent post of XoioX:
No weird in/out with precious data disk, no volume restorations and/or fixes ....
The NAS resurrects completly clean as "installable" (migration was + by previous broken procedure)
All done with *BASE* DSM v6.2 (DSM_DS3615xs_24922.pat) download from official page synology for DS3615xs and immediatly after, the latest update (6.2.2 update 3 in today case)
Double boot, as for everyone in DSM Update reporting, and all working fine!
Thanks again Bi0sh0ck, and happy XPEnology to all!
Byeeeeeeeeee