That's really a nice room. So much to love about it, including the windows we can't see on the other walls. I agree with colourhappy's point about negative space, and also with those who feel for other reasons that nothing is needed in the high spaces. The first and third photos posted by Rebeccamom123 illustrate keeping attention down in the levels people occupy. But those rooms benefit from a unified upper line defined by the architecture that you currently don't have. Rebeccamom123's middle photo shows wonderfully that that isn't the only way to a beautiful space, but this design is far more complex and required some talent. And I think yours also requires a little talent and training to make it look good. That TV placement may be necessary, but to me it creates a really significant visual problem. Since you're adding a mantle, etc., perhaps consider hiring a good designer to pull it all together? For instance, just because the TV support and lines come out at that point doesn't mean the TV itself couldn't be moved down (and sideways?) to a better position, adding architectural detail to disguise as needed. (Btw, strictly in the interest of feedback, one of those corner side walls looks big enough for your TV, or next size down, and I'd almost certainly have put it there. If it needs to be on the end wall to be seen from the dining and/or kitchen areas, and a TV in the kitchen's out of the question, of course that's the place.)

I do like the plant to the right and could move tv a little more to the left(toward corner). Love the height of art on left wall(draws eye upward), just add one more coordinating piece below it to fill in space. Would then add a horizontal decorative light/white piece of art above the tv for symmetry.

As far as I can see, there is nothing wrong w/ existing fireplace. If you are going to put tv over fireplace, I recommend this mount. There is a motorized version, also. https://www.dynamicmounting.com/product/down-and-out-manual-mount/

the charts for distance are to maximize the clarity. if you sit too far back, you're paying for something you're not using (you buy a 20" 4k tv and you have to sit 3 feet away from it to actually see the difference between that and 1080p). you can sit farther away but there's no need to... and at some point it doesn't work proportionally in the room. a tiny TV in a huge room looks off. regarding height, sure, you can mount it higher. no one will end up in the hospital. probably. but it's not ergonomically correct. you have the ability to do it right, so do it.