The basic premise of a surround sound system is to replace the room acoustics with the surround sound acoustical environment. To succeed at that room treatment is an absolute necessity. Given that your room is almost a cube, you are going to notice a huge difference from sidewall panels and bass traps. The ceiling will also benefit from acoustical panels, but at 20 ft high, you likely don't have the equipment or experience to do-it-yourself.
Our living room ceiling slopes from 12 to 15' with a lot of glass on the side behind the listener and 4 - 2X4 ft and 4- 2X2 ft 2" panels, covered to match the white ceiling absolutely transformed the room from unlistenable to pretty good. It even makes conversation easier.
Definitely match your RCL speakers and at a minimum add another Polk subwoofer. Replacing it with a pair of SVS (your choice of model0 would be even better.
Absolutely recalibrate your system using the Yamaha YPAO tool, then tune it by ear to your taste. Especially listen for speech intelligibility. Some TV shows (Billions, for example) seem to almost whisper their soundtracks, I suppose to make the script sound more conspiratorial. As a result it can be very difficult to follow if the room is just bouncing sound off all the walls. Again, in our treated room, soundtracks became much easier to understand - without raising the volume - after we treated and recalibrated the system.
Good luck!