Many thx for your contribution, thx to you 2 :)
I was thinking too about the full disk encryption. No problem to backup/restore current files.
Sadly Raspbian, and many others OS for the RPi, have not many options like x86 when you set up the system. That's why it can be hard to understand how to proceed with a running system, harder than set up a fresh Debian with the main useful menu with "use full disk encryption" option ;)
I'll read the links in your previous mail, it will be helpful.
Thx for your lights, to all ;)