Your reference frame in the Ethereal should be the same as the Material
In dynamics, all motion occurs relative to a frame of reference, since motion is the act of things moving relative to each other. When you Blink, you must also have a frame of reference.
Your "relative to the world" example still picks a reference frame of the world. You could also potentially define your frame of reference relative to the plane you're on. If it's a constrained single-world plane, the two are equivalent, but if your plane has planets, you could find yourself off-world due to the motion of the planet.
We know that there are no cases that force you off-world, so the question is how to decide what your frame of reference should be. In the standard case, your frame defaults to world frame because your velocity is fixed relative to it. It makes sense that we would use the bus for your case because your velocity is fixed relative to it.
A description of the Ethereal Plane is as follows:
The Ethereal Plane is a misty, fog--bound dimension that is sometimes described as a great ocean. Its shores, called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.
A description of Etherealness is as follows:
You ignore all Objects and Effects that aren't on the Ethereal Plane, allowing you to move through Objects you perceive on the plan you originated from.
It is apparent that you can move through objects on the Ethereal Plane, but you don't automatically do this, since you are not flung off-world, as discussed above. This adds credence to the theory that you keep whichever reference frame in which you are stationary at rest.
Unfortunately, there is no explicit RAW answer for you, but I believe any other answer has to make assumptions that are not RAW.
Ultimately, this is a DM ruling
Without a RAW answer, it is up to the DM to break ambiguities and decide how to set the reference frame. While I believe the above makes sense, your DM may decide otherwise, which is his/her prerogative. It is your right, however, to ask ahead of time how such a situation will be ruled and expect that ruling in all cases. Ask your DM to set a precedent or allow some player flexibility, such as using an Arcana check, as suggested by @MattVincent.