Jordan, again, Installing to Legacy on a modern UEFI system is like buying a new car and disabling all of the safety and performance features.
I understand you have CSM/Legacy Boot enabled because you have Windows installed in Legacy Mode to MBR. You are also getting a Windows Boot Manager error which means that UEFI is somehow still enabled enough it can generate that error. For this reason ONLY I asked you if you can disable UEFI in your BIOS - some can, some can't and the most you can do is enable Legacy or CSM.
In any case the issue being with Windows Boot Manager is a hardware issue with the motherboard so you should contact the Motherboard manufacturer for help with the error. During the first year they must fix or replace it, after that they may still help or you can ask in their forums. You can also ask in the best hardware forums Tom's Hardware here: https://forums.tomshardware.com/
What I would do in this case is enable UEFI, boot the Windows installation media as a UEFI device, during Custom Install delete al partitions off the drive, choose the resulting Unallocated Space and click Next to auto-create and format the needed partitions to GPT and begin install - this makes it foolproof.
You can keep Legacy CSM enabled to boot your other operating system if it's on another hard drive, boot it from the one-time BIOS boot menu key for your motherboard.
Report back results for each step so I know what else to suggest.
Feel free to ask back any questions. Based on the results you post back I may have other suggestions if necessary.
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