Theoretically, you can disable the Ubuntu submenu by placing GRUB_DISABLE_SUBMENU=true in the /etc/default/grub file and then running sudo update-grub. However, I couldn't get that to work, and according to the results of my DuckDuckGo searches, neither can anyone else. So, we'll manually edit the /boot/grub/grub.cfg file to fix that.

Look for the submenu line that appears just after the first menuentry item. It should look like this:

submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-f0f002e8-16b2-45a1-bebc-41e518ab9497' {

Comment out that line to make it look like this:

# submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-f0f002e8-16b2-45a1-bebc-41e518ab9497' {

Scroll down until you see the following line:

### END /etc/grub.d/10_linux ###

Just above this line, you'll see the closing curly brace for the submenu stanza. Comment out that curly brace so that it looks like this:

# }

Now, when you reboot the machine, you'll see the whole list of boot options instead of just the default boot option and a submenu. However, as things now stand, only the designated superuser can boot into anything except the default option.