I haven’t used plywood for a hive bottom board, but I have used ¾” plywood outside for other apps here in Michigan. No matter how many coats of paint I’ve used, the plywood starts to delaminate in 1 to 3 years. The pressure treated stuff holds up a bit better. I really don’t like plywood, the stuff doesn’t weather well in my climate and is heavy. Solid wood is a better long term investment.