Dolphin does not support DX12 any more. It was removed as it was no longer being maintained, and the "vulkan" backend offered pretty much the same advantages, and was being maintained and improved by active developers.
As far as I'm aware, there's not really much that the DX12 API required in extra features from the GPU over dx11, and dolphin doesn't really use any of those features anyway. The main advantage is a simplification of the driver model, allowing the possibility of lower CPU use for the same effects being rendered on the GPU.
So for dolphin, there's not really any reason to change GPU based on if it supports DX12 or not.
If you're not CPU limited, there's also not much reason to update to DX12 generally either. As I said, it doesn't really add many more GPU features, but attempts to allow apps to access those features in a more CPU-efficient manner.
As far as I'm aware, there's not really much that the DX12 API required in extra features from the GPU over dx11, and dolphin doesn't really use any of those features anyway. The main advantage is a simplification of the driver model, allowing the possibility of lower CPU use for the same effects being rendered on the GPU.
So for dolphin, there's not really any reason to change GPU based on if it supports DX12 or not.
If you're not CPU limited, there's also not much reason to update to DX12 generally either. As I said, it doesn't really add many more GPU features, but attempts to allow apps to access those features in a more CPU-efficient manner.