Thought I shouldn't start a new topic as it is kind of generic DX12 question: if I put my CBV in the root signature, does the driver version the data automatically, i.e. I wouldn't need a per-frame constant buffer resource?
You will still need a per frame constant buffer resource. You just won't need a per frame entry in a descriptor table for that CBV.
The only way to not need a constant buffer resources is to store constants directly inside the root signature, but you have very limited root signature memory, so you won't be able to store everything in the root signature.