Hi,
as a long time user of the previous model, the CP4, I partly regret selling it in favor of the CP88. Had them side by side for a couple months autumn 2019 and got my share of impressions from fellow musicians. Almost unanimously pro CP4, which is how I still feel, as well. At least in terms of the Grand Piano sounds.
Please, don't get me wrong, I like imy CP88 enough to keep it, mostly for the new UI and the capability to somewhat tweak the sounds to my liking. I miss the warmth and the plethora of better sounds the CP4 offered. Not only the APs, the basses (especially the Upright bass) and Pads in the CP4 were a lot better, IMO. I also liked the EPs more, at least I don't thinks there's a huge difference - not that they're bad in the CP88, they're definitely good and I enjoy the FX, in particular DRIVE and analog Delay a lot, but still, no huge reason to sell the CP4 if you own one.
So I guess sound-wise there isn't really a ton of progress since 2014, you have to listen to and play it for yourself.
Oh, and yes, the Nashville C3 is quite nice though, one of the better APs. I mostly like the CFX, the C7 and Nashville, hate the Bösendorfer. I had to tweak the Nashville and the CFX but I like the results for some situations. Oh yeah, and the Uprights are nice, indeed.