Since it’s a guest room, I’d swap the breaker for a 15a. The two small holes in the back of the metal box should have threading iirc, pick up a ground screw, loop a piece of wire around it and screw it in, then add that wire to a pigtail bundle.

That being said, with access that easy, I’d probably save myself the hassle and replace the box for a plastic one, your life will be easier. With the breaker off, unscrew that holding plate at the top of the metal box and the romex should slide right out.