Unless you plan to connect your "modules" to other people's modules, what you really want is a "sectional" layout. In simple terms, there are two main parts to any layout; the bench work and everything above the bench work. Mianne is certainly a viable option for bench work. It is super easy to assemble, modify and disassemble. Some think it's on the expensive side, but I think it's comparable to others if one compares it to similar quality lumber/hardware and considers the time involved in construction. I'm going the DIY route using less expensive 2x4s,etc., for my small 5x8x10 "L" shaped permanent holiday layout. I intend to have 7 legs on wheels so I too can move it out of a corner for maintenance, etc.
That said, any bench work put together with mechanical fasteners (as opposed to glue) can be disassembled, so what type of bench work you use depends on time, money and know-how. IMHO, your challenge will be break everything above the bench work into sections that it can also be disassembled somewhat easily. Unlike modular design, the bench work and top sections don't have to share the same footprint.
To that end, Mianne and other bench methods, like L-girder, are designed so that the plywood underlayment can be attached from the bottom. That way no fasteners are covered by landscaping and the "top" sections can be removed fairly easily by removing the fasteners below. As many people have shown, track laid over seams can be cut so sections can be removed, the hard part is the landscaping. Most people don't want to see seams all over their layout and there are a number of techniques that can be used to hide the seams between sections.
Multiple levels add to the complexity. Like your layout, mine is also going to have multiple levels that are not connected. Since no grades are involved, mu plan tis to divide the 2nd level into sections that can be lifted off. It will have upside down "L" risers, so the base of the riser can be fastened from below the first level and then the top of the second level can also be fastened from below through the "L" part of the riser.
Since my layout will be a permanent winter landscape, it will be kind of easy to hide seams on the "batten" I'll be using for snow and if a road crosses a seam, I may attach the road with Velcro so it can be removed. The point is to think of ways to arrange landscaping over seams so they can be less obtrusive than a saw cut. If a hill crosses a seam, you can put 2 pieces of wax paper or something between the 2 sections as you build the hill and then remove the paper after everything dries. Pieces, like rock outcroppings can be overlapped so the seams are mostly hidden.
