Good move on venting with a crack along the bottom of the door. it’s easy and functional.
As far as exhaust vent, the first thing to note is that an exhaust vent is really only to allow stale air to escape sauna, say, after a sauna session. Some purists like to vent during a sauna, but if you are using wood heat, there is enough fresh air pull, just by wood combustion.
But an exhaust vent is a good idea. You build a sauna once, so may as well put one in. Your instincts are correct. Put your vent opposite wall to your door, and I like about a foot from the ceiling. They’re easy to install. Drill or jig saw a 3 1/2″ hole in your wall sheeting. You can buy a vent with the exterior grill already in. Screw it to a stud and insulate around it. When you do your t&g cedar, just cut around the round metal pipe. I like to make a little vent control slot thingy using some t&g cedar, with a little wooden handle. I’ll try to take a picture for you sometime. it deserves a post.
PS.. If you have already built a sauna without an exhaust vent, no worries, just open the door when you’re done.