Grub-customizer is known to cause problems with booting for many users.
I suggest you remove it.
You can boot to a live install media then use a chroot environment to remove crub-customizer and repair the booting.
Full instructions for using the chroot environment are available if you need it.
This post has instructions but do not do step 8 or 9. Instead we need to repair what grub customizer has done after you are fully into the chroot environment with step 7.