Because you are getting the echo, I presume you are already drywalled? Did you do any of the sound control pretty much every does (decouple wall, GG, etc)? I decoupled ALL walls and ceilings.
I have an on open room but I'm only about 34' to the back. I used Brian Pape for a plan. I'd contact him if I were you. He can come up with something. It sounds like you have the right expectations going in. I just want the room to be as good as it can be. I just wanted to do everything I could, and then, as you say, it is what is.
I CAN say that with my front wall treated to 2 inches and my side walls now treated to 1 inch, I was in the space talking to my wife and I noticed a difference from the time I treated 1 wall to the time I treated both.
The back of my room is where the bar is, so I never expected any great sound there, but forward, in the theater area, I think it WILL be great.