A ramp isn't "necessary" outside the coop, however the way most elevated pop doors are set up, there's no way for a chicken to jump up and land inside the opening to go in. At the very least, you need to give them a landing platform to fly up onto, and then walk in. I use steps instead of a ramp as it saves space, as making the angle shallow enough for a good ramp can take up quite a bit of room.
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Stuff in the run is always a good idea. I have multiple roosts outside (simply branches shoved into the fence openings) and yes they do use them. The "obstacle course" as you call it is what many of us call clutter, and that's actually recommended: https://www.backyardchickens.com/threads/a-cluttered-run.1323792/
And the "unused space" above 2' is for your ease of access, as a human who is taller than 2'.
Maybe this will help clarify(?)
Take a look at and study the 2 previous post PHOTOS from @50-45-1 and @rosemarythyme
Roosts and/or ladders are in "open" areas inside the coop. Generally there is enough room for chickens to hop (using wings) or fly onto roost bars.
See how a Pop door is made? The small opening makes it harder for birds to hop or fly into the doorway. Some chickens likely would have no problem, but most keepers like to provide an easier way (ramp or steps) for them to get into/out of the coop.
