Danke für die Vorschläge @griven! Leider hängt er weiterhin bei der Installation, mit beiden Varianten.
Apr 29 19:03:58 iMac OSInstaller[550]: Machine appears to be AR capable
Apr 29 19:03:58 iMac OSInstaller[550]: Stash commit failed: 0xe00002bc (if stash was staged for an autologin user, this is expected)
Apr 29 19:03:58 iMac OSInstaller[550]: Pre-EFI tool stash commit: FAILED
Apr 29 19:03:58 iMac OSInstaller[550]: Attempting to flush install log back to IA.log