Ooh interesting...do you find yourself gripping with your knees or thighs? Because I imagine that would lead to inward toe pointing
I was about to say that you have loose joints. Tightness in your hips makes your toes point out, your the other way! The twist comes from your hip, not your knee, the loose knee joint just exacerbates it. You need to be thinking heel in. Try and bring your heel and point it towards the horses opposite hind leg. If you try it sitting down in a chair at first and move from toe in, heel out, to heel in, toe out, you will feel the hip joint moving to accommodate it. Then you just need to take that with you when you are on board and focus on it. It usually takes a month to 6 weeks to re train the muscle memory
I would second what Leo said, I have been having trouble with tight hips since an injury to my ankle and my toes look the complete opposite to yours.
Just out of interest what are your toes like if you ride with no stirrups?
Better - but my toes are still turned in slightly!
I have this same problem. I sit happily relaxed on the sofa, OH keeps on ‘feet, feet,feet’ as my toes are turned in (almost touching) and heels out. Feels natural to me, he says I am deformed 🙄. I have been diagnosed with hyper-flexible joints. My knees in particular are unstable. Exercises from the Physio have helped strengthen them. Hasn’t helped my position though!I've just had my second session with the osteopath. He seems to think that I have hyper-flexible joints and weakness/instability in my knees. I'm doing work on my knees to strengthen them, but it still doesn't really explain why I twist my heel out so much.
Hard to tell from that photo but to me it looks like it's coming from higher up and you are collapsing through your left rib cage, this in turn may make you collapse your left seat bone in and put more weight through your left stirrup which in turn may make your heel drop further and swing the toe in. I'd be tempted to try stretching up through your left side ribcage and checking you are sitting equally on your seat bones and seeing what the toes do then. I would also try to take more weight through the thigh on that side and less into the stirrup. Just my musings, have fun exploring !

I actually thought upper body wise you looked remarkably level, it is very hard to tell in a quick photo though. Cut some circles out of duck tape and place them on your hips and shoulders making sure they are level, then get someone to video you. Make sure you wear plain clothes so theres nothing to draw your eye etc. Its usually pretty eye opening!
My equi pilates teacher also does rider biomechanics sessions - is there anyone near you who offers something similar?
I'm just wondering how you put your leg on and whether a lazy one would cure you.
I'm lucky in the fact that though most of me is bendy my ankles are the opposite so they are my limiting factor, I suspect yours are bendy and therefore joining in with everything else.
I was thinking about this today while riding because I have differently wonky feet, plus the wobbly joints and I also tend towards the outside of my foot being against the stirrup.I suspected something like that, my oldie relationship is the other way round, I know what terror he is thinking before he does it (his main caregiver doesn't lol)
I don't know if this applies re having the foot right at the outside of the stirrup? https://murdochmethod.com/no-66-lengthen-leg/