Hadn't you installed grub-customizer when the problem first occurred?
To remove the grub-customizer files this guide may help - if you want to remove grub-customizer.
I read that grub-customizer often also changes the permissions of the original grub scripts so that it isn't enough to copy them back and you also need to set executable permissions again.
https://forums.linuxmint.com/viewtopic.php?p=2581615#p2581615
In an older thread I found the alternative recommendation just to remove the /etc/default/grub file and then to reinstall grub.
Afterwards you could set in grub the kernel 6.17 as saved.
I'm not sure what the best way is. I don't want to test grub-customizer on my system because it can mess up everything, and in a live session I can't test that.
Perhaps it is enough in grub-customizer to click on revert, but I read in many threads, that it seems not to be so easy to restore the original grub settings.