Possibly the metal plates, when painted, would not be as obvious to others as they are to you.

But for other solutions:

Put a sofa table behind the sofa and pull the end tables away from the walls. On one side or maybe even both, strategically place plants to cover the plates. They don't have to be directly in front of the plates, you just need a whisp of branch to obscure the plate.

Or use the entire area from the outer edges of the plates to create a wall of pictures. What you have now looks a bit awkward because you didn't sufficiently fill in the areas between the mirror and the collages. But a wall of items, to include a couple shelves, maybe a large plate, maybe a metal piece, would fill in the area nicely.

I would measure the rectangle you have now formed by the collages and re-create it on butcher paper on a floor. Then start placing items on the butcher paper until you have a design you like.

Alternately, change the furniture arrangement in the room so that you could place large objects such as book cases, a grandfather clock, or whatever your style is, in front of the plates.

I think it will look less contrived if you use two different ways to cover the plates rather than matchings items.

How about switching from the mirror to a larger tapestry -- that would indeed cover all the possible flaws -- and work into your Arts & Crafts/Mission-inspired decor??

Check sites like O----stock.com --- to see some tapestries that might work.

OR a larger rectangle artwork (OR a small artwork on each side) might work -- because the painting itself (think of fabric on a frame .... same idea ....) would be out from the wall slightly and help cover the boxes .....