I would suggest a stone hearth matching the plain stone trim but not as wide as the whole unit to reduce tripping hazard. Possibly with a black trim edge. Then a mantle shelf. the same stone would be great but probably too heavy. I think a 'live edge' timber mantle would be great and introduce an organic edge to the straight lines and angles. then paint the flat surface a neutral colour as others have suggested. display something with white/cream tones (candles?) and something with an accent colour on the mantle. if mantle too pricey a big artwork a little wider than the fireplace itself with some black in it or a black background chinese screen or similar. I have no commercial affiliation with them (just like their work) and don't know where you are based but Time 4 Timber Brookvale NSW could probably help you with a live edge shelf/mantle. good luck!

If you don't want to settle for either colour shade (dark or light) you can always do a mix: frame of your storage light and have the inside of the shelves is dark or the other way round to match your existing furniture. You could even go and make it a great feature wall. Maybe some the images below will give some inspiration. Another option is to "frame" the fire place area by putting some fire safe material behind and under (maybe in the exact height of your black sideboard on the left wall). Storage could be mounted to upper half of the wall. I'd probably try to leave the storage half "open" since the fireplace is kind of freestanding.

Scyon manufactured by James Hardie.

Fyrchek (fire rated plasterboard) is possibly overkill if all the specification asks for is non-combustible. Standard grade plasterboard of 12 mm thickness would also satisfy the non-combustible requirement. I am not sure what the bend/distort risk would be - as long as it is internal and not exposed to water it should be fine (although I am not a builder or expert). The Fibre Cement option would also work for you, but I am not certain of the best methods to cut it.