I have a Seymour XD screen and don't have the issues being described. I'm surprised that someone is telling you that it is the problem. What you might have is a woven screen (which the XD is) and LCD panel (screendoor) issue creating moire.
Is your Seymour screen material at an angle or straight up and down/left and right?
A good number of projectors around the age of your Panasonic have moire issues if the screen material isn't tilted/angled. I know that when I was screen shopping around 2011, I was warned about making sure that I tilted my screen material as so many projectors would be negatively impacted by the previously mentioned vertical/horizontal orientation of the weave. When I got my JVC, I discovered that it was not prone to have this problem, but I was covered in case a future projector was.
So that is why I ask.
Otherwise, I can see zero logical reason why the screen would be the cause of "pixelation" in the dark areas.
For under $5000, I would jump on a faux-k JVC. Unless looking side-by-side (like I was able to do at CEDIA 2016) with a true 4K, the JVC lines have been doing faux-K right and you would never know by looking at the image that it wasn't true 4K. Problem then becomes that the Seymour XD screen is great to 1080p, but not beyond. So you would need a new screen again (Seymour does have a 4K screen, but that would be a bit of $$$.
Epson would be on my hit-list as well.
Panasonic has kind of gone the way of Sanyo (which was my first projector make)... they were really impressive and had a great performance/price value in their day. Now they aren't quite the unique value that they used to be. Heck, is there anything newer from Panasonic than the 8000? Seems like it is still a current model even though they first shipped back in 2012. I am not sure that Sanyo is even in the business any more.
