You can get various thicknesses of OSB and other plywoods. I have used Fir also it’s comparable to OSB here in price. As for heavy you will require appropriate hinges for the door. I would use strap hinges for an exterior door.I was thinking so the window can be open all summer, so the inside is exposed to the weather and to keep the insulation from being exposed, but as a mite/mice haven I'm re thinking that. I think I want the bird hatch to open inward, so can latch to the inside of the big door.
The window....as a Dutch door, I would still have to double frame it so I can close it in the winter, yet leave it open at night in the summer.
Bracing....If I don't clad the inside and use something thinner than the OSB to lighten the weight, will it still warp? What would be sturdy enough that a predator can't simply lean against it and bust through?
I would use three of them.
https://www.homedepot.ca/product/ev...6yCHc-83QCLeGSWzHhg0owsj3TiFp8GhoCT1EQAvD_BwE