Last year we had a thread asking how far the Earth had traveled. Same basic problem of accounting for the different directions from the contributing components. I posted this trying to get at least a back of the envelope idea of how the motions stack up. (Trying to work out the distance traveled is harder as there are more large scale motions that contribute to aggregate distance. We can ignore them once we root the galaxy’s velocity to the CMB.)
The direction the Milky Way travels is not too far off edge on to the plane of the spiral, so where we are in the orbit around the galaxy makes a big difference to velocity. Our orbital plane around the sun is near orthogonal to the galaxy movement, so it’s contribution is more constant.
Choice of the CMB as a reference frame is probably as reasonable as one can be. So long as we are clear it is just a choice.