How to reinstall GRUB on an LVM system?

I have an old system installed Ubuntu 18.04 (desktop). It was partitioned /boot/efi on /dev/sda1, /boot on /dev/sda2, and / on /dev/sda3 (with LVM). The /boot partition was too small to install new packages. Therefore, I umounted the /boot and the /boot/efi partitions, and commented mounting /boot in /etc/fstab. Later, I reinstalled Ubuntu 20.04 on /boot, which is on the LVM volume.

The installation was successful. However, when I reboot the system, it boots the old /boot partition with the old kernel.

Summary:

1. /dev/sda1 is the old /boot/efi partition and /dev/sda2 is the old /boot partition.
2. New kernels are installed on /boot on an LVM volume.
3. The bootloader always boots the old /boot partition first.

How to boot with the new kernel? Should I type the commands:

sudo grub-install /dev/sda

I worry that the grub might be installed with the old kernel or break my GRUB menu. Besides, I should remove the old /boot/efi in /etc/fstab.

Thanks a lot!

Appendix:

lsblk

sda                       8:0    0   1.8T  0 disk
├─sda1                    8:1    0   512M  0 part
├─sda2                    8:2    0   488M  0 part
└─sda3                    8:3    0   1.8T  0 part
  ├─gs--vg-root         253:0    0     9T  0 lvm  /
  └─gs--vg-swap_1       253:1    0 127.9G  0 lvm  [SWAP]

/etc/fstab

# <file system> <mount point>   <type>  <options>       <dump>  <pass>
/dev/mapper/gs--vg-root /               ext4    errors=remount-ro 0       1
# /boot was on /dev/sda2 during installation
#UUID=d053f15f-12bc-4ae4-96e5-8d28aeb997ed /boot           ext2    defaults        0       2
# /boot/efi was on /dev/sda1 during installation
#UUID=7D6D-081F  /boot/efi       vfat    umask=0077      0       1
/dev/mapper/gs--vg-swap_1 none            swap    sw              0       0
UUID=7D6D-081F  /boot/efi       vfat    defaults      0       1