200x40 cm , 18mm thick, quite a large single board. Green timber will cup , any screws on the edges will be stress points and crack the board over time. Oak and sycamore are prone to splitting.
Do you want me to pop the guy at National Trust a message? if I recall he might still have waney edge maple near those dimensions, air drying over the last year.
By engrave I’m guessing you mean carving into ? Sharpen those carving tools, maple is on the hard side.
Theres a guy on instagram, Waney Edge City Timber, or something like that, has a yard just edge of north London, from the pics he’ll have exactly what you need and has a mill to cut it down to your size. He doesnt post prices, you have to message/call him.
For finishes -oak’s natural characteristics keeps out water, same thing happens with finishes, they dont get soaked in a great deal, they sit on the surface. A sand and reapplication after couple of years will make it look new again.
Green boards, think or thick will distort, some species slightly, others noticeably - direct sun accelerates/amplifies it.