archinstall 4.3's Installer.__init__ types target as Path and uses it directly (self.target / 'boot', etc.). Passing str(mountpoint) makes self.target a str, so every internal path operation raises 'unsupported operand type(s) for /: str and str' — most visibly during set_mirrors, which constructs target / MIRRORLIST.relative_to_root(). Drop the str() cast; pass the Path through as-is.