Can’t boot Windows: “Missing Operating System” – EFI Boot

I’m about 6 hours in and don’t know where to go from here.
I have one SSD Drive with an EFI Boot Loader and a Windows 10 System installed.

When I try to boot over the UEFI/BIOS, I only get shown one LEGACY Boot option: The SSD Drive. If I disable Legacy Boot, there are no Boot options. If I boot the drive, the PC displays Missing Operating System.

I tried booting into a live environment to repair the Boot Loader.
(I followed, among 2 others that did something similar, this guide: woshub.com/how-to-repair-uefi-bootloader-in-windows-8/ )
I also flashed a new BIOS. Fast Startup is enabled, but I also disabled it and it didn’t change anything.
After I followed the guide (two times), bcdedit displays the following:

K: Is the currently mounted EFI Partition (see below). The bootmgfw.efi file exists.
C: Is my installed Windows drive. The winload.efi file exists.

In diskpart, I tried to list some relevant things:

If you need any additional information, just ask. I hope so much you can help me!
I really don’t want to reinstall my system as I spent so much time setting it up over the last two years.