Run chroot-and-upgrade and apt install simpleproxy -y
Create a systemctl
While it’s not completely impossible that you can kill GRUB, destroy your partitions and break StartOS with chroot-and-upgrade (which is why its not recommended or supported), it isn’t possible to do so with the instructions provided.
Assuming you didn’t do anything else other than follow the instructions, this would imply that this would have happened regardless of what you were doing the next time you restarted the server, and suggests some kind of issue with the SSD. The only way back to reflash and recover.