Is the EHT "only" planet scale, or is it taking a second measurement 180 days later to increase the effective diameter of the virtual antennae to the width of the earth's orbit?
No, unfortunately those measurements have to be taken at the same time.
That said, as the Earth rotates the distance between any two pairs of antennas changes which can be used to add additional information to the images (those new pairs of measurements again have to be taken simultaneously).
From what I understand, this is less useful for looking at Sgr A* since the scene isn’t static and changes on a roughly 10 minute timescale.
