If you utilize a laminate Baltic birch, like Finnform or McFinn, made for use in building conctere forms, you will have nice prefinished weatherproof surfaces and it much stiffer than ordinary plywwods, so you can probably use a 3/4" thickness for your top and have plenty of rigidity. You will need to seal the edges where its cut for best weather protection, but otherwise it is near impervious to weather and reasonable abuse. I built a deck on the front of a small boat over 30 years ago from a piece of Finnform and it is still sound today. The edges have been resealed several times.
FWIW, Alfred Nobel's father, Immanuel, was the inventor of modern plywood & the rotary plywood lathe that peels the thin verneer from logs. He founded a company called Torpedo to make it.
