So, I actually got it working 
A user over at badcaps.net, where Sweet Kitten linked the first image from, has build a LENOVO BIOS AUTO-PATCHER for Supervisor Password Removal.
So I downloaded that tool and pretty much followed the instructions lined out in the thread using the unbricked_old.bin that @lfb6 had uploaded. So basically:
1. Run the tool on the unbricked image
2. Flash that image to the chip
3. Boot up and follow instructions ← this gets rid of the supervisor password
4. Flash the original unbricked image to the chip again
5. Factory reset BIOS (which is now accessible without a password!)
It does look a bit shady though, ngl, so I’m not sure yet what to think of it and if it didn’t introduce some unwanted guests or something.
But: It did work. After that, the machine did boot normally without any errors or warnings. Bonus: BitLocker was still in place and accepted my old password, so I was able to recover everything and everything is working as if nothing ever happened!
Thanks so much to both of you, @lfb6 and @Sweet_Kitten !