heyy nice studies mate, you may not like it, but I think your perspective study is pretty nice, I'd actually love to see more XD I like how you populate em - that said I also struggle to make that first stroke on any perspective or landscape work
As far as shading goes @mitsuki-youko really hit it out of the park with those tips! I'd also add that choosing good reference material is essential, when the plane changes aren't easy to note it can be much harder to paint so I recommend looking for more dramatic lighting scenarios to make it easier as you learn. For me it's similar to what Mitsuki mentioned of block in, shading and smoothing. It varies depending on the app im using, in procreate I love using the smudge brush, in photoshop I find that using color picking and opacity on my brush works better to just make those gradations. As for layers, on photoshop I do tend to place the lineart as foreground with multiply and low opacity as a guide for the painting phase and sometimes I'll fill in the area to paint and then alpha lock it (if im not lazy XD) works well for me either way as I like keeping it a bit messy, free of limitations and thus achieving a more dynamic and painterly result (at least that's my goal) . It also helps my brain to paint the background with some tone other than white, it could be the actual background color, some other color or just some gray to help me better analyze the values as I paint em
dang that was long, also, glad to hear everything went well with the procedure!
cheers!