The startup menu is gone - Can`t install other OSes (iMac 12,1)

[meteora_xz]

Hey! Not sure if this is the right section of the forums, but hope for some help anyways.

So I have been testing out different Linux distros on the family's old iMac 2011 (12,1). After a while, I settled on Linux Mint. Unfortunately, I was unable to adjust the screen brightness on the machine, regardless of which distro I was running. After a while and a lot of googling I came across thishttps://askubuntu.com/questions/1295423 ... brightness guide and edited some parameters in Grub. Screen brightness was now functioning, but now I am not able to boot into the startup menu of the Mac. This goes for all the other recovery methods as well like Shift+Alt/Option+CMD+R, CMD+R, and holding down the Alt/Option is not doing anything anymore. Is this possible to fix? Any help is much appreciated!