OK, partial success. I created an insync.conf file with (basically)
'su -s /bin/sh -c “/usr/bin/insync-headless start” ’
and insync now launches when the computer boots up (not just when I log in). It does synchronise even if I don’t log in (or if I log out). However, this process that runs in the background means that I don’t have access any longer to the GUI. In order to recover this access, I have to quit the headless process and launch insync in a console. But then, of course, it’s a process linked to my current session and, when the session ends (I log out), the process stops synchronising…
In order to make sure it keeps on synchronising after I log out, I have to quit the insync process I started to access the GUI and launch “service insync” to make sure the (session-independent) insync process of insync.conf is running.
So it’s a partial success. I thought that the “insync-headless set_autostart yes” would be the way to go, but it does not really launch the process “when the computer boots”, only when I log in…