Seeing your love of geometrics and clean lines, would you consider adding some interest and texture with a geometric pattern tile, topped with a beech (is that the material in your alcove units?) mantle shelf? Very modern, but also very custom and interesting. At the moment, the space is quite sparse (but gorgeous!) and this might help warm things up a bit? There are also some amazing turkish tiles around now (2nd pic) that would add even more colour, but that might be a bit much for you? I would love it though!

I have a 26x16 ft large room which badly needs updating but am concerned that with the big stone fireplace, I can't get a more glamorous look. I'm really bad at this and need help

I think a rectangular fire could look jammed in between the bookcases, unless you can find a short one... Maybe you could simplify the fireplace by changing the backing board for a pale colour (not beige) and changing the surround for a really streamlined boxy shape. You could keep the mirror above. You say there's a lot going on - if it's possible to rearrange the items in the recesses in the two bookcases so they are more symmetrical, eg have some books on both sides, some candles each side and so on, it would give a "calmer" look? Another thought - normally shelving either side on a fireplace is recessed in the alcoves whereas yours are in line with the fire surround. If you had a fake full height chimney breast built and brought the fireplace forward (would only need to be the depth of the shelving) it might make the shelves look less dominant

Any ideas how I can make this fireplace look a little nicer and more modern? Thanks