Table of Contents

GRUB

GNU GRUB (short for GNU GRand Unified Bootloader)

reboot to different kernel once

Obligatory changes:

/etc/default/grub
GRUB_DEFAULT=saved
# Comment or disable:
# GRUB_SAVEDEFAULT=true
update-grub

Working example

# get menu ids:
egrep -i "menuentry '|submenu '" /boot/grub/grub.cfg
# Set grub env using chosen ids. For submenu use '>'
grub-set-default 'gnulinux-simple-21b9a8a1-48e8-4c4d-8420-6c97870f2702'
grub-reboot 'gnulinux-advanced-21b9a8a1-48e8-4c4d-8420-6c97870f2702>gnulinux-5.3.18-3-pve-advanced-21b9a8a1-48e8-4c4d-8420-6c97870f2702'

If everything works well, remove Debian kernel. 

apt remove linux-image-amd64 'linux-image-4.19*' 'linux-image-5.4*'
update-grub
grub-set-default 0

specify boot entry

Grub2/Submenus Display grub menu and options without rebooting? Grub: reboot to specific kernel

Main menu boot entries can be specified by

by name

grub-reboot "Advanced options for Ubuntu>Ubuntu, with Linux 4.2.0-16-generic"
reboot
 
#
grub-reboot "Other platforms>Minix>Version 3.4.0"
reboot

by menu id

by number

# set first entry
grub-set-default 0
 
# boot 2nd menu next time (boot once next time)
grub-reboot 1
 
# boot 7th submenu from 2nd main menu (boot once next time)
grub-reboot "1>6"
 
#
grub-reboot "1>0>0"