Interesting design. As you said you're pretty far along. Personally I would limit myself to at most two levels about 28" apart. The bottom level would protrude out further from the wall below the top (as it does in your proposed design.)
I've seen a lot of multi-level HO club layouts. None of them had more than three levels. O gauge trains are taller. When you take into account the height of rolling stock, buildings, and a proportional "sky plane" / horizon (look at some landscape-orientation photographs of real trains) you'll see why I feel like you should have 24" between levels. I also think 24" is the minimum suggested height of a backdrop in O gauge, with heights of 36" or even 48" recommended.
Another thought... your room is large enough that you could create a helix in one corner of the room, and use that to move between the levels, instead of the ramps. 24" might be too much for a ramp, but you can get that with four turns of an O72 helix. I've tried designing helixes myself, and a typical footprint might be 7'-by-7' or 6'-x-10'. Usually this track is hidden in a mountain or behind a false wall, and not scenicked. You can even use it for "staging," i.e., hide a train inside. I don't personally have the room for one, but you do, so consider the advantages.
It's your layout but I believe you'll enjoy it more with two fully scenicked levels, instead of four stacked tight like an O-gauge club sandwich.
