There is a hook automatically regenerating the grub configuration after a kernel installation or update (by-the-way the command would be sudo update-grub).
Unfortunately, no prioritization and a strange situation. If for whatever reason you cannot use the tool garuda-boot-options, you could use this manual procedure, which I used many times, and was also successful for others.
The problem here for you is that at that time, you would not be able to know the “position” of the linux-lts kernel (this you will see the first time you reboot), so my suggestion in this case, since you need only the linux-lts kernel would be that you also remove the linux-zen kernel before rebooting. So the linux-lts would be the only one and default.
sudo pacman -R linux-zen linux-zen-headers