The ISO you are using 240702 appear to fail with the Nvidia driver installation by the mhwd script.
I suggest you fetch the latest ISO using the sourceforge link mentioned above.
Setting the firmware to hybrid allows the system to boot with free drivers.
Setting to discrete you need the boot with proprietary mode.
It seems the command grub-install referenced in the setup logic is missing
/usr/sbin/chroot: failed to run command 'grub-install': No such file or directory
This means the ISO is incomplete - not your fault - I don’t know what went wrong - I believe it should have been addressed back in May when the stable ISO was done after Plasma 6 release.
Before running the installer - open a terminal and sync the package install-grub, then run the installer
sudo pacman -Syu install-grub