Mallard 4468's advice is good. With lumber prices skyrocketing post-Covid, it is good to find benchwork options that depend on engineering rather than materials for their strength. After building a supposedly portable layout of 1/2" Baltic birch and 1x3 bracing, I found that the 50" x 60" table was far too heavy to be portable as intended. I now use 1/4" plywood and 1x2 bracing (I glue all my bracing with carpenter's glue).
Having built a largish modular layout (although in HO) with standardized "dominos" I found that when we moved my modeling interests had evolved sufficiently that I didn't want to try to save any of the layout--and so we left an entire 24' x 18' layout for the new owner. A later L-shaped switching layout (MR Sept 2004), built on L-girders with plywood only under the track itself, simply went to the dump when the time came to move.
Contemplating your trackplan, I would give serious thought to extruded foam panels on a thin but deep L-girder system, with no intent of moving it when it came time to downsizing. You WILL need access hatches inside your loops, and those are easier when they are blocks of foam. The only reason you need 2x3 bracing or plywood thicker than 3/8" is if you will be forced to climb onto the layout to perform maintenance. For foam construction, see dagryffyn hobby's Youtube channel.
I'm reminded of Alfred E. Neuman's famous line, "Learn from the mistakes of other, 'cause you'll never live long enough to make 'em all yourself." I'm old enough now that I've made most of the mistakes, and I will never overbuild a layout again.
