Hollow door built of marine grade plywood is going to last as long as a solid wood door but be VASTLY lighter. Simple torsion box construction will make it solid, stuff it with whatever insulation and then lay on the other side and voila, thick, solid feeling door that weighs a LOT less than solid wood.
Its not just hinges that need to be stout, the leverage on the wall or post that those hinges are mounted to is a lot with a 6+ foot wide door, think what you can do with a 6' lever and your weight, imagine what a 1/2 ton (say a solid oak door) can do?
Funny I saw this thread as I have been thinking about a round door for my olive tree, its massive and overgrown, the branches reach the ground in what must be a 30' diameter circle and its a wonderful open dark space underneath and I have been thinking about adding a door to get under its leaves. I don't need to insulate so I am going to make the door with false hinges and have it pivot at the 1/3 mark, might even counterbalance it so there is little if any stress on the frame.
Kirsten Dirksen did a video of an awesome hobbit hole in Washington State, owned by Kristie Wolfe. Lots of not permie construction tech but it could be modified to be more permie. Paul may have to take a trip over there and see how the door works.
The above door appears to be a re use of the end of a cable spool. They are constructed by nailing the boards to each other at 90 degrees to each other on opposite sides. The grooved circle on the inside of the door is where the boards that made the core of the spool were inset. If both ends of the spool were used with insulation in between with a hinge as I described before that pivots on the center of the door and latches with a cam bolt through the center pivot so that when you turn the center knob a weat is lifted ub out of sill on the bottom of the door and pin pulls down out the lentil at the top of the door then the door could be pulled out of the wall and pivoted on the center of the door and the door post so that it can swing to the side with the outside of the door facing you.
The Door post and center pivot on the inside of the door are made of steal pipe that the hinge pipe can rotate around. The hinge Then is a square of pipe that has the open ends vertical so that they can rotate around the door post and center pivot with the same pipe above and below to hold the hinge at the right height on the post and the center pivot to the door.
I think it would be visually interesting to be able to turn the center knob and pop the door out of the wall like pulling a stopper out of a bottle.and then swing it aside. When the door is closed no hinge is visible on the outside and if the center knob can be removed as a key it makes it very difficult to break into.
One idea I've heard of is to use metal 2x4 framing filled with rigid insulation for the core. Then you layer thin strips of wood on the outer edges. After that you put 1/4 inch plywood on both sides, then paint it to look like wood planks. This makes a very light door with better insulating qualities than a solid door. I would use a knob in the middle and levers similar to ones from a manusl garage door opener.
This looks like a job for .... legal tender! It says so right in this tiny ad: