Can't install Big Sur after booting EFI from USB (MBP Pro mid 2012 15" ⟨MacBookPro9,1⟩)

Hi everybody,

i'm trying to install Big Sur on my MacBook Pro mid 2012 15" (MacBookPro9,1).

Host system is Success' href='/2020/12/install-macos-catalina-on-supported-pc.html'>Catalina 10.15.7

1st Try:

1. Installed OpenCore Legacy Patcher
2. Downloaded oldest available Big Sur Version (11.7.10)
3. Transferred to USB
4. Patched USB
5. ⚠ After choosing EFI, no installer was shown

I found a tutorial claiming that 11.6.5 is the last Big Sur version with a dedicated installer

2nd Try:

1. Searched for 11.6.5 and found a ISO on archive.org (not sure if im allowed to link it)
2. Mounted 11.6.5 (v16.8.00) ISO
3. In terminal sudo /PathToMountedIso/Contents/Resources/createinstallmedia --volume /Volumes/usbstick
4. Open OpenPatcher and patch USB Stick
5. Booted from EFI, clicked install Big Sur
6. ⚠ Installation couldn't be finished, with a message saying that the installer was corrupted somehow

3rd Try:

1. Downloaded an even older Big Sur 11.2.3, this time not an ISO but a pkg file and installed it
2. Opened OpenCore Legacy Patcher and created install media via "Create macOS Installer" and chose 11.2.3
3. Patched the USB stick
4. Booted EFI
5. Started Installation
6. ⚠ Can't choose internal SSD as target, saying something like "this machine doesnt support Big Sur"

Tried the 3rd Try again with 2 other USB sticks, as the OpenCore Troubleshooting page says that the USB can be a reason, but with the exact same result as in the 3rd try

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General Info:

MacBook Pro mid 2012MacBookPro9,116gb RAM (2 x 8GB CRUCIAL DDR3L 1600 MHz PC3-12800 SODIMM 204-Pin)1TB SSD Samsung Evo 840

All USB Sticks are Partitioned as GPT/GUID, not MBR

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anybody got any tips on what went wrong?

Thanks in advance!

best regards