When I try to add the grub launcher as a boot section, it gets removed from the list after reboot (as I read this is caused because the RW_Legacy firmware not having access to NVRAM). I also read that I have to edit the ChromeOS’s grub menu, but I don’t know how to do it. I have to go open Boot Manager and boot to grub from the file. This problem is specific to grub as other bootloaders override ChromeOS’s grub menu.
The easiest way would have been to install the Debian bootloader to the partition where the ChromeOS bootloader is, since the ChromeOS bootloader doesn’t do anything useful.