The solution to this issue is to run the below commands in order:
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=EndeavourOS
sudo grub-mkconfig -o /boot/grub/grub.cfg
Thank you @Madic, @2000, @joekamprad, and @dalto for helping us all to arrive to this solution!
Hey @ibana and @dalto, can we have this post accepted as the solution and close this issue?