hey,
I solved that problem for the ‘protostar’ demo by using shell geometry so that the ray trace could start very close to the surface, otherwise the trace had to start at the camera plane for every pixel.
You could start with a giant plane mesh at the correct elevation and eventually upgrade to a sky dome that is actually spherical to get some nice perspective. check out the zero dawn cloud paper if you have not. Keep in mind even with ridiculously awesome optimizations, they still had to use a special buffer that only rendered 1/16 pixels each frame and then reprojected old frames for the rest. So their sky was animating very slowly a few pixels at a time. So it is very unlikely that anybody will make a fullscreen 3d volumetric cloud shader that runs very fast without special optimizations like that, or at very least using severely downsized translucency.
And maybe you can look into using variable step size based on viewing angle. That should be fairly simple and you may find that it only takes ~3-4 steps at shallow angles and more like 64 or so at steep angles.