Hi @LVTTL
As @marcel.tx pointed out, you need to adapt the file /etc/fstab.
Change the first line to this and you will get a read-only file system:
/dev/root / auto noatime,ro 1 0
We will also investigate why this is not set by the option EXTRA_IMAGE_FEATURES_append = " read-only-rootfs".
Best regards,
Jaski