Is there a way to do a full backup of internal LE (OS & data) to sdcard ?
No, you only need to backup the data storage partition. LibreELEC has a backup & restore section in the LibreELEC Settings add-on. It will store the backup file in /storage/backup. After that, move that backup file off your SD card into a safe (network) location.
If you want to use network locations, make sure you have created manually mounted shares before starting the Backup routine. Otherwise, the default backup location will still be used.
Otherwise you could ssh to your kodi system and convert the installtointernal script to an installtosd script. It's located at /sbin/installtointernal
