great advise.
I would also go with a construction similar to the picture posted earlier. my father and I built a large version of this about 6' x 12' for a model railroad setup. 1.75x 5.5 oak aprons, 5 in sq poplar legs with the corner braces and 3 lags per leg on the diagonal as shown. the top was lighter, but maybe not by much with all the 0-gauge trains, the bridge, buildings, transformers, etc. top was 3/4 baltic with 1x4 oak rails around the edge and a "rung" at each plywood joint. about 11 feet of span between the legs and no sag whatsoever. that table has been around about 20 years now and is still flat as ever, I think it may qualify for some kind of flatness spec like a surface plate.
I think you have a good plan even if the slab weighs more like 400 lbs. I would add a center "truss" of a slightly taller dimension, could be solid or a couple thicknesses of ply glued, and about 3 ladder rungs in the frame. no ply infill, not necessary and does not really add any structure. I think dominos would be great.
I/we often way overthink stuff like this.