Restoring GRUB configuration after reinstalling kernel (CentOS 7)

I have CentOS 7 running on a very old Linode compute instance.

I thought it would be reasonable to upgrade to CentOS 8 following a guide I found, but the process has not gone smoothly, unfortunately. Worse, I have already followed an instruction to remove all the kernels.

Now I have needed to restore the previous kernel from vendor repositories, and I am still struggling to ensure proper configuration for GRUB.

After reinstalling the kernel, I confirmed the kernel and initramfs having been placed in /boot. Then, I ran grub2-mkconfig -o /boot/grub2/grub.cfg.

However, checking grubby --info=ALL, I see no boot entries reported. I also find no entries by direct inspection of the generated file.

After some investigation, I have learned that Linode instances may rely on a customized /etc/default/grub, but I have no means to verify that the current version is the one I would want, or to restore to one more suitable. The file as currently found is shown below.

How can I ensure that my boot configuration is correct, allowing the instance to boot the kernel installed on CentOS 7.

GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=0
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="console=ttyS0,19200n8 net.ifnames=0 scsi_mod.scan=sync crashkernel=auto rhgb "
GRUB_DISABLE_RECOVERY="true"
GRUB_TERMINAL=serial
GRUB_DISABLE_OS_PROBER=true
GRUB_SERIAL_COMMAND="serial --speed=19200 --unit=0 --word=8 --parity=no --stop=1"
GRUB_DISABLE_UUID=true
GRUB_GFXPAYLOAD_LINUX=text