I think (?) I maybe making progress. Today I went to a second hand shop I mentioned above and bought these things for the mantel. Look okay? Then, I bought these vintage billard balls and put them in a glass container I already had for the coffee tables. Very small investment for everything. Next is the TV area. I'm telling you I have really struggled with accessories in this open plan. Next trouble area.

I agree that painting the back wall behind the TV black will help hide the empty space. If I had to place a TV in this space I would probably center it. You are already way over viewing comfort height and a few more inches won't make a big difference. I would also do something to camoflage the TV. I'm not a huge fan of mirrors, but they do have mirrors that you can place over a TV and they look like a mirror when the TV is not on but the TV picture is displayed when the TV is on. Other options include art or doors over the TV. Here are some ideas: https://www.tidbitsandtwine.com/the-great-cover-up-7-ways-to-disguise-your-tv/

I would paint lower cabinet doors black and insides of bookcase black. Add a couple simple white and gold accessories along with a group of books light backs. Add simple look on mantel on side of TV. Add black woven basket for toys beside fireplace. Add stack of birch logs to fireplace for decorative look if you don’t use it often. Do not put personal photos on this wall. Better in gallery arrangement in master br or in hallway. You can enjoy more when you can get closer to them. Guests aren’t interested in your personal stuff. Keep it simple since your sweet baby will be running around before you know it!

I feel your pain. We have a 1930 Tudor Revival house and a fireplace very similar to your except for white brick around the fireplace opening (where you have more of a decorative surround). We have lived here for six years and the previous owner had the fireplace restored, but he put on a white, glossy trim piece as the mantel, just a few inches wider than the niche. I've worked around it for 6 years, but we are now looking for a raw, vintage piece of natural wood to put in place of the trim piece. We were just down at two salvage stores in Seattle today looking for the right piece. We just need to decide if we want to have it go past the sides of the fireplace or be as wide as the bricks below. Those niches were appropriate for the 1930's, but they are impractical for today. I want to display artwork and seasonal items on my mantle and not be tied down to the small niche. I'll send a pic when we decide what we're doing. I love our tudor revival -- still have the leaded class windows from the 30's.