What you have drawn is a project of massive proportions. Unless you have a club to help you with it, it will take you many, many years to complete. In addition, the way you have the room set up, the focal point of the layout is the helix.
In his book, Mid-sized and Manageable track plans, Iain Rice gives a good discussion of the the relationship between time/money/space and in essence, he advises people not to bite off more than they can chew. Joe, the previous poster has a layout in proportion to your design, but he has people from all over the country coming to run on it and he has help working on it.
In our area, there is another layout of that size. likewise, he has help–several people come over every week to work on it. In essence, it has become a club layout, if not in name.
I’m not saying you should base your decision on what I have to say. Although I don’t have the space you have, I have a basement to work with. I started with a two level layout and realized how many compromises I had to make to have the second level. what I decided was to have one level and put twice as much effort into it–make it really nice. I found that I could have all the design features I wanted–and better at that, just not as many sidings or as long a run.
In any case, a double deck layout does not look as good as single deck. There is too much visual distraction until you are up close.
So my advice, take it for what you paid for it:
Make one nice level.
Loose the long center peninsula. Use smaller 5’-wide peninsulas going north and south if you want.
Widen the benchwork along the walls to 30", Provide depth of scenery and vary the track work. Make extended branch lines and service industries. Run a double mainline so you can have trains running opposite directions while drinking your beer.
Put a spectacular yard on the long wall (or an