Being vertically challenged (over 6 ft. tall in a world generally made for people shorter than 6 ft.) i can tell you that you can spend the next 10 years trying to figure out the best multi deck separation, and not reach a conclusion.
My finding is that there isn’t a sweet spot and it’s really a crapshoot. You give up something thinking you gain elsewhere, but really there’s no win-win situation at all.
The best advise I’ve read somewhere was that when you are building a layout, you are building it for yourself, first and foremost. Everyone else must adapt to your setup or go elsewhere if they don’t like it.
Many people build layouts low for their grandchildren, or at the height to accomodate their friends, or whatever other reason, except for themselves. Honestly, to me that’s not being noble. And you are not being selfish if you build the layout for yourself as a primary user. All these other people will not be there most of the time - you will so it has to work for you.
Another thing to consider is aging (of a person). Right now we don’t have the luxury of exoskeletons, yet. So the knees, feet and hips have to support you operating your layout…Or you build a single deck and work from the chair only, if you can’t stand on your own today, or might not in the future.
Honestly, don’t bother with open grid framework, it is redundant and not a benefit at all in this case.
I suggest Rubbermaid double track shelving system instead. You can buy 48" or 72" tall uprights and then pick from a selection of various depth brackets (8-24" deep). You will be up and running in less than a day, requiring probably 3-4 uprights, depending on the setup (more on that below).
For a surface, a 3/4" (of better quality) plywood, and if really needed (for cutting into the ground for scenery) a 1-2" pink foamboard.
Your middle deck will be the deepest one, while the upper has to be shallower, or it will interfe