So a couple of other ideas you might want to try. Samsung makes their Frame TV that shows your favorite artwork when not in use. https://www.samsung.com/us/explore/frame-tv/highlights/ Only downside is there is a separate 14.3" x 4.5" x 1.2" IO box that needs to be hidden out of view somewhere and still connect to this TV. You could actually hide the IO box in the recess and then install the frame TV over the top assuming the mounting hardware will reach past the edges of the recess. Another option to look at his Seura TV which is the same idea but a mirror when not in use. https://www.seura.com/products/smart/ They have two flavors-- a bathroom mirror type where even at 2-3" from the mirror you cannot see the TV. The TV quality is not as good but a better mirror. The second is the Living Room mirror type where you cannot see the TV when standing 3ft or farther from the TV. Not as good of a mirror but a better quality TV. On this one be careful on the 4k side as they are 4kUHD compliant but not HDMI 2.0, HDCP 2.2, or HDR compliant the last I checked. That's a bunch of tech compliance but bottom line is you want all that in your TV if you want the very best picture available these days.

I just went through the same thought process. There is a difference, tvs should be at eye level when sitting, art should be at eye level when standing. If you look up the average eye level for a 65" TV the center of it should probably be about 41" from the ground. If it were a painting the center of it should be at 57". So it's 16" different. I just mounted one and I decided to optimize for TV watching not art. Partly that's because I previously had a TV mounted high enough to be out of range for kitty cats and it was pretty stratospheric. Uncomfortable to watch to be honest. So anyway. To balance the Frame TV on the wall I will need other art work. Right now the TV is alone on the wall and it looks wrong when it's in art mode and you walk by it. It doesn't look that bad though. To be honest this is one of those decisions where you probably win either way. It won't be that uncomfortable to watch if you mount it at art level. And it won't look that bad if you mount it at TV level.

I think if we had a wider fireplace insert it would make a big difference overall. That is something I stated was a misstep already. Our TV is recessed in alcove. The mantel and surrounding materials are approved for fireplace. It’s true there was significant work needed to have this meet code. I actually can’t imagine the mantel any higher as it is already at my chest height when I am near it. It really is so personal. The only thing I would shift is our fireplace insert size.

I would go smaller. It's going to look off if the TV is larger than the firebox below - too top heavy. Chose your TV size the same way you would if you were putting art up there. And also, be prepared for the cognoscenti here to show up and berate you for putting your tv up there. You will hear it will wreck your neck, your tv will explode from the heat, and people will walk into your house and scream in horror. Yes it doesn't matter that multiple other people here can disprove all of those statements....you will still get sniffed at in derision. :)