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Huh. So could one use that as a standard frame of reference?

Yes and no. The laws of physics do not pick out a preferred frame of reference, and they should be symmetrical under lorentz transformation, but the fact that we live in a cosmological universe starting with a big bang breaks that symmetry in practice.

It's similar to how the laws of physics are symmetrical under translation and rotation, but the presence of matter around us means the surface of the earth is not the same as the surface of the sun.

So, while we might use the CMB as a standard reference frame, the laws of physics should not.

Maybe, kinda. It's not obvious that it's more privileged than centering a reference frame on the Earth or the Sun. The CMB is maybe just an artifact of the Universe's history (like a Sun-centered one is an artifact of ours).

If we didn't have any CMB radiation floating around to measure, everything would probably work exactly the same. Maybe you could measure all the motion of all the matter in the visible universe and measure the motion of the "center" relative to us.

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