If you plan on a 24' wide garage, the rafter span will be 12'. 12' with 3/12 pitch will be 36" rise plus the height of the rafters. 12' + 36" = 15'. What size of rafters did the architect use, they must be very big to have a roof height of16-3.
You could use 1/2" plywood in the middle and 3/4" T & G wood on both sides of the plywood which would make a 2" thick door. Seal the back and all edges of the T & G before assembling, that should help keep the wood stable.
You must work in construction. Very impressive to raise 2x6 walls, 50' long, and 12' 6" high with what appears to be a crew of four. How thick is the Isoclad and does it have any sheer value? Great looking build.
When my son remodeled his house, he had to have several things engineered. One item was how to raise the height of some walls. Engineers solution was to frame new walls on top of the existing and connect new and exiting top wall plates walls together with threaded rods. Do not remember the...
I use to install metal strips called sill risers. They attached on top of the threshold and were about 1/2" high. Cut the bottom of the door off 1/2" and reinstall the drip cap. This was many years ago and I don't know if sill risers are still available.
Remodeled a house and customer wanted a room to keep his gun safe in so safe would be out of sight. Access to the room was in the garage and to conceal the door I put a 4' x 8' sheet of peg board over the door. Peg board was hinged for access to the door, looked like just a sheet of peg board...