That brass is there to provide the necessary clearance between the firebox and combustible material (your mantle). This makes it difficult to build a backerboard braced with a wooden frame in the empty "space" you've identified. Again, that airspace is required for safety and code.

If you'd like a new look, have you given any thought to painting or replating the frame? There are many metalic paints available to give it a different feelt. Alternatively, If you could identify the manufacturer, you may be able to just order a new front in a different style.

I suppose you inherited someones design choice? Some prefer 'brassy'. I don't. As recommended above, call the manufacturer and ask about options. The surrounding frame could be removed, sanded and painted a matt black, but if that is yuck, they will tell you what you need to do to fix it. And how to prep for tile. You have many options and should search for answers. Traditional fireplaces from an earlier era were built with first brick and mortar, then a tile surround, then a surround of often a stone or wooden mantel. You are seeking only decoration and not what was often needed for safety with tile. You have an insert. All the rest is a frame of decoration. Do what makes you happy but it is not traditional. If it drives you nuts, find a less 'flashy' option from the manufacturer. Or if it is just glass doors, it can be removed and inserted with a tile surround and smaller doors....