https://wiki.tnonline.net/w/Btrfs/Profiles - DUP mode protects against data or metadata corruption, but not disk failures.
Alright. Looking at that link carefully I understand. My brain was not fooling me. To have protection against data corruption it is necessary to lose half of the storage space. So that statement is half true. With that system you lose the same capacity as with a Raid1. To do that it is preferable to create a Raid1 and for the same money you also protect yourself from a disk failure. Also don't stress the disk so much.
So for now I'll stay as I am, if I'm forced to format I'll probably do it in btrfs just for better compatibility with OMV, but nothing else.
Thanks for the info anyway.