With spherical projection you need a single image with 360x180deg. With DEMs it doesn't matter as long as they are lat/long projection. You want at least 16-bit for elevation and it doesn't take too long to get to a 4GB TIFF. A 36K texture (0.01deg/px) is 1.2GB
SRTM15plus data is 0.0046deg/px. To load that I have 4 images covering 180x90deg weighing in at 1.8GB each. If we could specify size and position as a ratio of 360x180 (assuming the image is also 2:1) we could load these as size = 0.5 and position lower left at 0,0.5 0, 1 0.5,0.5 0.5,1 but I'm not sure how difficult that would be code.
Or support BIGTIFF?