Hi there, welcome to the forum.
Check if this helps (I have no experience at all with iMac’s, macOS and the like):
https://wiki.archlinux.org/title/Mac#Installing_a_boot_loader_to_a_separate_HFS+_partition
It looks like it is suggested using the macOS Disk Utility.
There is also an AUR package ( mkfs.hfsplus tool in the hfsprogs package), but it won’t be availabe in a live USB.
Anyway, from the screenshots I see that the error arises in the last step of Calamares, when the files have already been installed and the system just needs to create the ESP partition and install GRUB (I don’t know if there is anything else actually, I am not a developer).
Maybe you could use the live USB, and from the Garuda Welcome use the “Chroot” button, to see if you can get into a terminal on the installed system. Then you could try to install that package and format the ESP partition and then install GRUB.
I’ll add a couple of other links I found, but I don’t know to what extent they are useful (maybe in case of subsequent problems with efibootmgr, called as part of the grub installation):