Really large table bracing

Hi after some advice about building a large 16 seat table

the dimensions of the apron are:
3.5m x 1.1m


making it entirely (apron and top) out of a hardwood called Palawan wood side is:
140mm x 25mm

It seems sturdy just as apron/frame so I'm worried about it sagging over time. So going to put 3 boards of the same material along the short axis as I'll need something to screw the top boards to

will that be enough to stop it sagging or can I screw something on the long sides like a piece of 3 or 4mm steel plate to try and stiffens it


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