I have an open living room with a 16ft peak sloped ceiling - not as large as yours.
The 4 panels mounted high (need a ladder) effectively controlled the echo alone.
I also added a FreeStand panel in front of a window that unfortunately sits between a front speaker and the couch. (otherwise I do not recommend absorbing first reflections) There are also 2 - 6" panels taking care of a 'booming' room mode. So that is a total of 7 panels. The weird layout of the room doesn't allow for many DIY solutions, unfortunately.
Book spines work well as a natural diffusor as well as providing some absorption. You are thinking right if you are already planning some DIY solutions - the world does not sound better simply because of acoustic panels. The normal furnishings of a home can be more than adequate when used appropriately.
I also think GIK acoustics Art panels are just awesome and are definitely spouse friendly! They use Dye sublimation printing so it fully soaks each thread as opposed to a surface-only coat application that might peel or flake. I see they also have (finally) started offering them in multiple sizes - 242 was all that was available when I bought about 5 years ago. They will print any image as long as you have the rights to it - I think I paid an extra $20 or $30 for the image....