Advice needed to go beyond intermediate level

[Sarskate]

You said that you've been trying to impose a Chinese only rule, which is excellent but often they don't last the distance, unfortunately. A Variation on this, which usually does stand the test of time, is language hour (a set time/times every day/week) dedicated to languages. A Meal/brunch time is usually a good one to choose as you have time to formulate a response in between mouthfuls and everyone is generally a bit more relaxed.

Another one I recommend to parents looking to build bilingualism (I'm a language teacher and edu consultant, though in French not Chinese) is a language room. Have one room in the house where, when you enter and interact in the that room it must be in Chinese. For young children we usually recommend a play room of sorts where it can become a fun place they want to be. For older students, the room should change every 1-2 months to allow for more diverse language and vocabulary.

The third way would be in conjunction with anything else you decide to do. Have a regular movie/games/activity night with other learners or native speakers, with the rule that Chinese must be spoken. It will take a bit of pressure off from your wife being the only regular practice you get, and you will pick up heaps of colloquial expressions and everyday structures. Just don't forget the wine. It is surprising how quickly you get into language when everyone's had a glass or two