30 inches is ideal FOR ME to reach and work on standing on the ground when the layout is about 40 inches off the ground. I am about 5 -6/7. I have deeper areas that I can easily reach standing on a stool. Another thought is that I find 4.5 inches looks too wide for track spacing. I base mine on appearance and ability to reach between tracks and I use 4 inches for parallel tracks.
An advantage of around the walls is that you can do a lot of buildings as just facades, maybe 1-3 inches deep against the wall rather than doing a complete building. These usually require kitbashing but take up little depth. That can be a disadvantage too.
You can make sections deeper, much deeper as mentioned above but you will need to crawl up on the layout to work on them. If you do this, I suggest keeping the track no farther away than 30-36 inches from the front where you can reach. Track generally rquires much more maintenance than scenery in general.
