An update on my side: everything is working now!
I tried your suggestions but they didn't work out unfortunately, but I'd like to share my workaround. I tried a different more manual approach since booting the system from the Unibeast USB (without multibeast and post-installation stuff) has always worked.
Steps I took:
- Followed all installations steps from your guide up till step 7.
- Instead of using the Multibeast post-installation method, I now post-installed using the manual Clover EFI Bootloader onto the High Sierra SSD.
- I selected all available drivers from the Clover Bootloader installer, and added additional .efi drivers manually by mounting the High Sierra EFI partition with EFI Mounter v3. The ones I added manually are: "apfs.efi" , "HFSPlus-64.efi" and "OsxAptioFix2Drv-free2000.efi".
- Copied the config.plist file from the Unibeast USB EFI Partition to the High Sierra SSD EFI partition.
- Copied over all kexts from the Unibeast USB and added "FakePCIID_XHCIMux.kext" , "FakePCIID.kext" , "Lilu.kext" , "USBInjectAll.kext" , "WhateverGreen.kext" , "X99_Injector USB 3.kext" .
I did not install VoodooTSCSync, is it actually necessary? And if so, what's the method for manually setting it up?
System is working very well right now, and there are no boot issues anymore. For some reason, it seems like multibeast did some strange things to my system but I have no clue what that might be.