fstab looks good to me.
this has something to with the issue, but the fact that you reach grub rescue shell means something is(maybe) working. you have to make sure the grub you drop into is from the manjaro and not from previous linux installations. i’m not at all familiar with grub prompt but try and see which partition it refers to in that shell.
other than check “/boot/grub/” contents in manjaro-chroot environment. mine looks like ;
ll /boot/grub/
total 56K
drwxr-xr-x 6 root root 4.0K Dec 16 08:41 ./
drwxr-xr-x 5 root root 4.0K Dec 16 08:41 ../
drwxr-xr-x 2 root root 4.0K Apr 27 2020 fonts/
-rw-r--r-- 1 root root 11K Dec 16 08:41 grub.cfg
-rw-r--r-- 1 root root 1.0K Apr 27 2020 grubenv
drwxr-xr-x 2 root root 4.0K Nov 22 05:30 locale/
drwxr-xr-x 3 root root 4.0K Apr 27 2020 themes/
drwxr-xr-x 2 root root 20K Nov 22 05:30 x86_64-efi/
you might not have all contents, but as per archwiki the reason why you drop into grub rescue shell is because of absence of grub.cfg