Using Cyangmou's video ref I came up with this:

The unexpected thing about it is that making his top half lean back actually makes him appear MORE balanced, which is contrary to how i would think about it in my mind.
But when I tried this pose physically I realized why.
Sure hes leaning back, but really hes pushing his hips forward.
Feeling it out this puts alot of pressure on the pelvis and abs.
The top half of the body is basically "hanging" from the lower half, which is doing all the work to hold this guy up.
I found this interesting so I thought about it some more.
The generic center of gravity in the body is relatively low, and even lower in women.
It seems that keeping the upper body vertical or leaning forward can be associated with sitting down, which is why it feels like she is falling over.
When we go to sit down we lead with our ass and lean foreword, using the low center of gravity to gravitate us to the chair.
The opposite happens if we were to notice a thumbtack on the chair, the upper half leans back and we lead with the hips forward gravitating away from the chair:

Of course these are exaggerations, but in art that's a good thing.
Really these could be even more exaggerated and work just fine.
Especially as part of an animation.
I'd like to do an edit but don't have the time at the moment.
Cool design.
With some more work you'll end up with something really nice.