I know that TB III is waterproof. But will it stand up to freeze/thaw
cycles in an exterior application? I have a 75" brick opening in which I am
going to replace a patio door, which requires a 72" RO.

I am going to tear out the nasty-looking framing in there now, and redo it
with some nice poplar. I was thinking of taking 2 1"x8" by poplar boards
and putting a nice graduated routed edge on each of them, and then gluing
them together, such that I'd have 1.5" on each side of the opening, bringing
it down to 72".

Will the glue hold the pieces together? Of course I'll tack and screw them
as well, but I want them to look like one piece with no separation. The
area is covered by a large overhang, so it only gets splashed rain from the
patio, hardly ever direct rain.

Should I consider a polyurethane glue instead?