Glad to hear that worked @Luke_Mahoney! You can get more detail in the Bing overlay by setting the overlay’s Maximum Screen Space Error property to a lower value. You may want to keep the max texture size to something reasonable when you change this, though. 4096 or 8192 at most, probably.
Cesium OSM Buildings uses “additive refinement”, which means that big, important buildings are in big tiles covering a large area. Smaller buildings are in smaller child tiles. When we zoom in close enough to show the small buildings, we continue rendering the big tiles with the larger buildings, too. This is different from “replacement refinement” where we switch from the big tiles to the small tiles as we zoom in. Now, because those big tiles are subject to the texture size limit and screen-space error computation, and because they remain visible even when we’re zoomed in close, the overlays draped on large buildings are unfortunately going to be blurrier (when zoomed in) than the textures draped on small buildings in small tiles.
Which is a long way of saying that Bing imagery draped on Cesium OSM Buildings is right now mostly useful for adding a bit of color for viewing buildings from a distance, and it won’t be possible to make the textures perfectly sharp.