I think the 77 would look great in that space. We can't see the full width but clearly there's plenty of room on both sides of the screen for speakers. I'd try to get it to sit a bit lower, or get a tilting mount that has a lot of movement tilting down.
I'm using this and it seems well built for the price. I wouldn't put another 100 lbs on it, but the tv mounting bracket seems more than adequate for a 77" LG OLED TV.
All that said, if I'm the one that makes you spend a bunch more money, I'm sorry but I know it would be worth it! Getting a larger CX and tilting it down wouldn't cost a ton more than a smaller GX, though.