You need steel brackets.

An 84" span with 3cm estone and a 23" depth is too much to not have additional support. If it is not supported the middle will sag. It's a little late for inconspicuous steel front to rear, however, you could screw a custom 2" steel square tube painted flat black to the pony walls set back 6" or so.

I am a fabricator. I agree you need additional support. You might consider a 1-1/2 or 2 x 4" steel tube painted black running parallel to the long edge about 8" back from the edge. This will provide adequate support and be simple to add since you have the wall supports on each end.

We extended ours so we sit on both sides...13" on both sides.