And also means more initial cost to buy drives I don’t really need right now.

My worries are to have to backup a pool of, lets say 30TB (I don’t know if I every have this many data), and need first a storage solution to upgrade my original storage. But maybe it’s better to let my future self to solve this issue.

Right, I didn’t get into detail much but as I wrote my post and as I got feedback, the build and configuration is improving already.
I already use TrueNAS Core on a Server witch I use as my Network storage (NAS) but what I’ll probably build is this 12 bay case into 6 HDDs NAS for daily use and 6 HDD backup of the NAS and other data. Both pools built as RAIDZ2 (RAID 6). Then I can use the Server for other things (game server or whatever). But to start I’ll think about your suggestion to start with RAIDZ1 (RAID 1) and two disks, if I need to add more, I’ll just add another two disks in RAIDZ1 and add them to the same pool.
I’ll edit my plan to the end of the post for others easier to find.
Thanks a lot and Cheers