Isn't that going to be way too much? I mean, just for settling 10 cities you get 55 Culture and Food. Next city gives you 11C/11F, then 12C/12F, etc. That seems a bit broken to me. Get a GW after that and it's another 11C/11F total. Just this part of the UA alone seems broken, it never will taper off as you say - the advantage will only increase as the more cities you have, the more your gain is - just settle/conquer another city, you get slightly more. Then settle yet another, even more. The yields might not scale by themselves but as it is, China can always expand or get a great work, and expansion's benefits will always go up, scaling by doing so, Not only that, the UA has a few other elements as if the first thing alone wasn't enough, such as free super-WLTKDs. Old vanilla France +2C per city UA didn't scale at all and fell off, sure, but this one constantly scales and contrary to what was claimed, will never taper off. It scales better than pretty much any other UA in the game.
I dunno, it seems to me like it will be more broken than pre-nerfs Arabia about 1 or 2 years ago which IIRC got 4C/4S for each Historic Event, and almost everything was a Historic Event. China also seems like a bit braindead of an UA too, to be honest. Just settle a city and you get the best yield, Culture, and lots of Food everywhere. Wasn't getting bonuses from creating/having Great Works supposed to be the Assyrian and Egyptian unique thing, too, which is why some other civ didn't get that sort of thing in some other thread when it was proposed? I definitely remember that discussion. Considering that, this UA seems like a very weird choice.
Settling cities for benefits is also Spain's niche, and Spain has a much worse a tradeoff as it only gets a small, instant amount of Food and Faith (not for the capital), inferior yields that will almost instantly be in inferior amounts. If you want any sort of truly meaningful gain from that UA, you must conquer - which is fine, as it is fun. Here is China's another problem - just doing something you're meant to always do grants such a high reward as inevitable 55 C/55 F in classical era (if you went Tradition, it might be somewhere in the 70C/F range). Not only will the UA be nuts as it'll start very strong and only gets stronger, the Paper Maker also supposedly receives a buff.
I'm sorry, I do not like it at all. It seems like the new China is Spain, Arabia, Assyria, India and old vanilla France put in a mixer. It's going to be too strong IMHO.
My suggestion:
+2C per city. Receive a WLTED in your empire for every city and wonder acquired. Every WLTED permanently increases the Food in your city by 2-5%, up to 20-30%.
Basically, old vanilla France but way stronger. Falls off, but has a lasting impact. You do not necessarily have to construct wonders, only get one through any means, like conquering a city with them, which means it works for every difficulty.