Stick with a 5.1 in this room. Try to get an amp with preamp output for the front two channels so you have the option of a separate 2 channel amp in the future and keep options open for a 2 channel system for audio only. Shop last year models. I got an rx820 for about 75% to 60% of new price. Spend less on back channels and more on the center channel - that is where most all the voice is sent. You need a sub. I agree with you about a 77" non-projection tv. May be too big for that room though.
For speakers, I would keep an open mind about other brands. I don’t have much experience in that price range. Put the rear channels in wall or ceiling. There may be other brands that offer better sound than something form a big box store. And, maybe look at used too. I would look at bookshelf speakers for that room front L&R plus the sub and a great front channel. Good luck. Fun stuff.
Finally, I much prefer the wall behind the TV to be dark and consider some sound dampening material in that room or it is going to be reflection city.