Boot drive doesn't show up in Open Core Menu after Catalina installer triggers first reboot

I'm about to converting my HP EliteBook 840 G2 (Intel Core i5-5200U, Intel HD Graphics 5500) to a Hackintosh using Open Core and Catalina as OS. I'm using https://dortania.github.io/vanilla-laptop-guide/ as a guide and it all functions well until the installer reboots for the first time. It can't restart in one take because my BIOS sais that the CMOS Checksum is invalid and so it has to reset the BIOS settings (which doesn't even happen, because all settings stay the same) and so it restarts one more time. After that the Open Core Menu doesn't show any additional drives, only the USB Partition where the installer lies on.

Disabling RTC Checksum checks or other RTC fixes don't seem to help.