Can you chroot again into the system, then see if sudo pacman -R plymouth is really removed?
Then please try a sudo pacman -S install-grub then again a sudo pacman -S grub
It should run automatically then, since there was a topic about a installation script
It should find your installation, otherwise, you would need to make sure, your efi drive (nvme0n1p1) is mounted on /boot/efi and install grub with --efi-directory.
That simple reinstallation seems like a shortcut, if it works.
If not, you need to do from your live-usb after chroot sudo mount dev/sdb/nvme01p1 /mnt/boot/efi
and sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck
and update-grub and reboot.
You can also look with inxi -G if you got any video driver installed.
If there is nothing, you can autoinstall it with sudo mhwd -a pci free 0300 or nonfree, depends, what you prefer. It looks like, there is a problem with the video driver.