grep menuentry /boot/grub2/grub.cfg

Copy the name of the OS you want as default
then access the /etc/default/grub file as root

sudo gedit /etc/default/grub 

Replace whatever is currently being given to GRUB_DEFAULT with the OS name you want as the default (enclose the OS name in “”)

Then run as root:

grub2-mkconfig -o /boot/grub2/grub.cfg

…and reboot to test

I don’t think this survives a Kernel update, but the process is not onerous, and no sacrifices to the gods are required.