Not sure about the second cactus, but time, and shading, will tell. Maybe take the background back to a single gradual slope. Also conflicted on the distance solution. Adding saguaros the middle ground helps with the receding scale, but the transition between layers feels crispy. While I have diverted on the subject matter, I am trying to follow along with the spirit of the illustration lesson.
This artist calls for a lot of sharp-edged brushes and alpha lock. While I do prefer inking brushes for blocking, I usually like a little more patchiness in the background line quality. This solution is to add an airbrush of fog between layers to fade it out, but I’m not crazy about the desaturation of the night sky or the loss of contrast with the jellyfish. Maybe if I alpha lock the land layers and apply the fog as a clipping mask instead. If nothing else, I suppose there’s always the blender.
based on Beginner’s Guide to Digital Painting in Procreate: How to Create Art on an iPad® by Max Ulichney, et al.