My questions :
Do you think that these choices are correct in relation to my use ?
On the possibility of using a USB boot drive, I've been using good quality USB sticks from the beginning. With the flash memory plugin installed, I've never had an issue.
In your case, I'd use SNAPRAID over traditional RAID5. For the same investment, one drive, you get bit-rot protection and the ability to restore deleted files and folders, among other benefits. (This is above and beyond the ability to restore one drive.) If you're looking for a common mount point, mergerfs can give you that (called UnionFS in OMV).
Regardless of the storage method / type used, if you want to keep your data, back it up. That's not to say you'd have to back *all* of it up, but I'd back up the data I want to keep. Otherwise, truly, it's just a matter of time. My choice is to backup everything (100%) to a separate and independent device - a backup server.