Weight capability can increase with closer cross member spacing. Talk to Tim about that. He can arrange any assembly configuration. I would think that 16" spacing of cross members no longer than 30" should prevail.
However, my primary concern would be to design a layout that would not require any need to get up on it.
If you need to get up on the layout, IMO, there was a design oversight. None of us gets any younger or more agile.
However, before you make any bench work commitments look at the web site of custom model railroad benchwork. An OGR advertiser. They make custom curved flowing framework to suit any available room.
I am considerable larger than GRJ and crawled all over his platform that has wider support spacing than I would design, but there was no discernible flexing with my weight over the 1/2" multiply Birch.
