Years ago I salvaged a large mirror from a bathroom I was working on. One entire wall was mirrored (oh how very 80s) and I was able to get one large piece off the wall without destroying it.
After having it sit for a coupe of years, leaning against a wall, I finally got around to mounting it on our bedroom wall.
Simple mirror hangers is all that is needed to mount a mirror. I then added some 3/4″ trim around the perimeter then some simple window casing, A coat of paint and I ended up with a large beautiful mirror at a fraction of what it would have cost to buy one.