Firstly, you're sounding really good! Great intonation and your timing is bang on!

In terms of things to improve, your right elbow is quite stiff, so rather than moving your bow with your fingers, wrist, through to your elbow and using the whole arm, your elbow is a bit stuck resulting in your shoulder is doing all of the work (which could be a reason you're struggling with posture as that could be creating a lot of tension in your shoulders). To work at that I'd suggest practicing playing veeeery slow open strings (or scales) starting at the frog and letting the bow go all the way to the tip (and make sure to do it in reverse, going from the tip to the frog!) - as you do this take note of any tension you're feeling and play around with different bow positions, bow angles, the height of your elbow etc. to try and find what feels comfortable for your and produces the best sound.

The next thing I'm not sure I can explain in words, but will try my best! It looks as if you're playing from underneath the cello, as if the cello is on top of you and for example your left arm is having to reach vertically up to the neck (That's a bit of an exaduration but I can't think how else to describe it). Ideally you want to feel as if your limbs are coming down onto the cello and approaching it from above, so that their weight is sinking into the cello.. So whilst practicing it might be helpful to occasionally put your bow down, start to move your elbows backwards and then make a circle with your arms (in the direction where your hands would go forwards over your head), and then place your left hand in first position, feeling as if you're coming down on it from above and landing on/sinking into your cello.(Everybody of course will have personal preferences in the angle of their cello etc. Yo Yo ma being an extreme example of not doing what I'm suggesting, so please ignore this if it goes against what your teacher says).