Colour wise and movement, the Panasonic is still extremely good in these areas. I would say that if your not bothered about 4K, then maybe look at Epson's 1080p models as these still use a evolution component which you Panasonic uses as the LCD tech in side your Panasonic is Epson's very own LCD tech with a few tweaks.
In order to bring some of the performance back, the
EH-TW6700 really is a must, but as to how much better it will be, this one is a tough one as the sharpness is easily better, but as we all know, there is more to a projector than sharpness alone
Your alternatives are DLP which point you to products like the
Optoma UHD51. Do check your measurements though as the lens shift is like the Epson EHTW6700 is not the same as your old Panasonic PT-AT6000