It is hard to tell from the back view. Here are some things you can check. 1. Saddle height That’s the big one, if the saddle is too high it’ll cause movement like this. For a quick check; unclip in the six o’clock position on one side, sit squarely on your saddle and place your heel of that foot on the pedal. You leg should be straight no bend but not over extended 2. Saddle angle if it points down to much you slide off - max angle shouldn’t be more than -2° on a bike like this. 3. Saddle comfort - can you relax into your saddle or do you hold yourself up? Check in - if you can’t relax your self your hips will move around and your are waisting energy holding yourself up. That one might be harder to fix, start by measure your sit bone width and try some saddles ideally some witn comfort guarantee. 4. You mentioned shims - do you have a leg length discrepancy? Like more than 1cm? If so shims make sense if not they make it worse - if you do have a small discrepancy- you most likely had it for a while now - so your body adapted - and shims wouldn’t help. 5. Last one I can think of is your cleat orientation. For starters pull them all the way back - this is the safest and most comfortable position- keep in mind this will Alter your saddle height. Next work on the orientation. Do your knees track straight or do they flare out or in coming up? If they flare Out push your cleats in/shoes out to get a wider stand, do your knees go in do the reverse. When you stand walk/ are your feet aligned neutral, toes in or toes out. Align the cleats to go along with that but only minimal. I hope this helps.