I have answered, twice.
So what would your solution be for a relentless chewer? Yes, training. But at some point during that training, you have to eat, sleep, shower, go shopping. Or heaven forbid, work.
So what do you do with a 12 week old puppy, who you are redirecting everytime it chews, following training advice, do it it all to the letter when you can’t have eyes on them? Shut them in a room with no furniture, skirting boards or wires?
Puppies don’t chew due to lack of training, they chew like babies chew, because they have teeth coming through. Most puppies aren’t districtive chewers who will always do that - it’s just while they are teething.
Thry would feel a lot more secure in a quiet, dark crate that resembles a den than behind a door which they would probably get distressed and chew. They would soil that probably when they got distressed and be uncomfortable. Dogs do not soil crates where they sleep (unless left for hours on end, which obviously, is wrong).
You cannot watch a puppy 24 hours a day.
Also, when crate training a puppy you have a routine! For feeding, water, play, toileting, sleeping. I don’t think you know what crate training is or is used for.
Some people do it, some don’t. I didn’t with my four previous goldens - they were in a very small utility room with a baby gate, but same same. We had a routine for the training. There were not given the run of the house as puppies.