I can easily do a 10th on the keyboard..I have enormous hands. I play everything very quickly when left to my own devices.
Knowing that I like to pound rather than play, go swiftly and not very accurately, I've asked my teacher for pieces which develop a finer touch and require more accuracy. Consequently, we are working on a Mozart sonata.
Any tips on achieving that wonderful light "Mozartean" touch, particularly in the left hand?
Wow, I can reach a 9th if I strain as much as I can ... and I don't have particularly small hands, just average female hands. And a good stretching capability, I think. Well, well, it normally works for me anyway. I avoid Robert Schumann.

OK, so your problem is accuracy. I see no other solution than you working with that, just as you say you do now. The key word is PATIENCE, I think. You really have to stop yourself and investigate, from the very very fundamentals, what you are doing with your fingers, your hands, elbows, shoulders ... your whole body. You have to experiment about the legato, try just three notes or something, and watch your movements carefully in ultra-slow motion when your fingers go up and down. In legato, should the keys "meet" somewhere in the middle, or should the first key ALMOST be all the way up before you start pressing the next one? This is what you have to work out. Experiment a lot, try to achieve different sound effects with different techniques.
And I am not pretending to be an expert here, I just summarize what I recently learned from someone who REALLY knows about these things. What I was told, which I found was surprisingly wise and also surprisingly elementary although I had not thought much about it

, was that you should concentrate on solving ONE issue at a time.
Which means it is a very bad idea to try to learn a new piece, speed it up, work on your accuracy and your legato technique all at the same time. So, for legato exercises I recommend good ol' scales and arpeggios, or maybe some of those dreaded Hanon exercises ...
Also remember your wrist, especially when playing legato. For the right hand the "rule" is: when you go upward on the keyboard, your wrist describes a "happy smiley". Which means that your wrist should be in the highest position, same level as your knuckle or even a bit higher, at the "turning point" when you are about to go down again. And from there, you draw a sad smiley.
An alternative description is that your wrist is "slipping in a spoon". It is also a good way to achieve a good balance in a chord, because this means that your fifth finger will be the last to leave the chord, thus giving a slight emphasis on the highest note, which is normally what you want. So, you make this spoon movement also on heavy chords, especially final chords in a piece. Personally I think this gives a better, more rich sound.
For the left hand, you do the same but in reverse direction, not very surprisingly either.
ALSO note that I am not talking about horribly exaggerated movements here, rather delicate ones. Nevertheless, a wrist which is too stiff will produced a stiff sound because you let your fingers do all the work and few people have this control.
I also learned a way to get a more accurate and elegant sound in ornaments and trills, which I suppose will do the work also in Mozart pieces. If you are to play an ornament with your right hand, let's say you play 3-2-1-2, you must observe the finger which is coming next. So, when 3 goes down, you "prepare" 2 but lifting it and straighten it right over the key which it is to press. And then, boom, it goes down very accurately - and presses the key in a correct curved position, of course. And while this happens, you have already lifted your thumb a bit, etcetera ...
Again, it is not about exaggerated movements that will cause you pain or something. And at first, you must of course work slowly enough to maintain full control of what you are doing.
I learned this little technique when I complained about my playing being clumsy and blurry in the ornaments. Yes, this did it for me. OK, so it was Beethoven and not Mozart, but I guess it will work fine with Mozart too ...
A third way to get more accuracy, which I am practicing myself very much right now, is to move your hands very rapidly and then have them prepared in the correct position in time. A typical mistake I often do is that I play a chord with my left hand, then there is a pause while the right hand is doing something fast and complicated and THEN I realize that the left hand is supposed to play another chord, one and a half octave down from the first position, and I don't move it there until the very last moment. And as I am a slow learner, I normally spend some microseconds fumbling around like "umm, where was I supposed to go nooow? Oh yes, there!" and it goes without saying that the result is very inaccurate ...
So, the right thing to do, of course, is to move my hand immediately after I have played that first chord, and then let it wait in the correct position. This requires a bit more work early in my learning process, but it pays off later.
Well, you can try my pieces of advice and it they don't suit you, then they don't. They worked for me, at least.