Grub2 comes with a tool called grub-reboot(8). Using grub-reboot you can reboot the system into a preselected configuration. This way you may choose at runtime — before rebooting — which grub menu entry shall be booted into. No manual selection in the boot menu required. Entering grub-reboot and hitting <TAB> (on Ubuntu systems with bash-completion, that is) you can easily display all available menu entries. No need to look for menu entries in /boot/grub/grub.cfg.
~# grub-reboot <TAB> Memory test (memtest86+) vg-root) Memory test (memtest86+, serial console 115200) Ubuntu, with Linux 5.6.0-1021-oem Ubuntu vg-root) vg-root) Ubuntu, with Linux 5.6.0-1021-oem (recovery mode)
The bash-completion might show some quirks when it comes to LVM enabled systems.