Your experience with grub-customizer is no different to other Manjaro users
Grub conflicts with grub-customizer
grub-customizer package
Grub conflicts with grub-customizer - #24 by cscs
grub-cusomizer is crap software. dont use it. it doesnt work well. it breaks things.
Manjaro GRUB issues from this package go back many years
Grub-customizer - possible caveats - archived.forum.manjaro.org
You need to find and fix the syntax error so update-grub can update GRUB correctly
do you have a file /etc/default/grub.pacnew ?
That may contain configuration changes that need to be merged with /etc/default/grub
It may also help you spot any configuration error in the file