It is a terrible design choice is the biggest issue. Even the manufacturer didn’t try to line the niche walls in it fully.

I’m actually not too worried about the niche as it will mostly be covered by a giant TV. I’m more concerned with the parts that will be exposed—like the edges of the fireplace wall itself.

You chose a horizontal install vs the vertical install of the carefully angled and cropped website photo...which also doesn't show or suggest trying to wrap the tile around corners.

The installer should have refused to install the tile in a horizontal orientation & a more experienced one would have.

Going to be a bear to grout as you can see the problems grouting in the first photo from the mfgr.

With a TV going into the niche, I hope you are prepared to install remote speakers.

This is not good tile work. Would suggest you have it all removed.

I agree that the tile should have been insatlled with a cleasn edge where the corners meets then the schluter would have been almost invisible the niche in the web photo was topped with a very different piece of schluter my guess maybe even stair nosing. IMO no way to fix what you have without a redo . How was the schluter fixed since it needs to be installed before the tile .This is what the corner should look like but of course with those tiles done vertical to have a clean edge.