Shred an internal or system partition There are quite a few apps that lets a user delete the media or data partitions so it won't be recoverable - by re-writing "junk" data over and over again on the free storage space.

Still, is there any way (app, ROM, tool, etc.) that does the same process for the system partitions? (assuming that root or custom ROM/recovery image is required for such process)
Eventually I solved it by connecting the device to my pc, running adb shell and then running su in order to grant root privileges.

After that, to list all the partitions, I used the df command - and in each partition I generated a file big enough to fill all of the free space with random data (generated using the command dd if=/dev/urandom of=junkfile.txt bs=1048576 count=AmountOfMBs, as explained here).

Note that this method still doesn't shred the space that's currently in use on the device, so before going through this whole process I did a factory reset and then signed-in with a dummy account.