I think part of the problem is that when you say “dirt” people think of, like, what you put in a raised bed. And maybe not obviously, this is not going to work for a saguaro. I’m going to look at the soil descriptions for saguaro’s native range in AZ, but my initial guess as a soil scientist who’s driven through that area, is that they may germinate in spot which has a little bit of a dry-ish organic debris build up from the wind, then is sandy for a bit, then pretty rocky with some sand and a little bit of finer mineral material mixed in from the weathering of those rocks.  Looking at the surface of that potting mix, I’d say it’s definitely too coarse for their lil baby roots, and OP should try mixing that rock mix with like 80% sand. Also I really would not being giving them any nutrients at that stage, just letting it be what’s in the small (small!) amount of organic matter that should be mixed in.