You can now download an AppImage from the releases section.

It uses grub-reboot for that function.

No. It just reads and parses the contents of /boot/grub/grub.cfg and updates /etc/default/grub

Yes, it only reboots into the selected entry ONCE without changing the default one.

You’re tempting me to give it another try after over a decade. I used to daily drive Opensuse with KDE 3.5