Just to point out, in that picture you posted, it just honestly goes to prove that you should not have to 'reach' for your stirrup. Just because it is Dressage does not mean that you must have a super long stirrup leather.
The rule of thumb for riding is drop the stirrup, and let your leg hang. The iron should touch the top of your ankle bone. Not the bottom, or under. Then, if jumping you should shorten them a hole or two to create more of an angle.
The way your leg is sitting tells me that you are reaching too much and they are too long. Your toe has pointed outward, and the back of your calf is on the saddle, instead of the side of your lower leg. Your heel is also pointed in towards the horse, instead of sitting flush with the horses side as it should correctly be in Dressage.
With a shorter stirrup it would be easier to wrap your leg around the horses barrel correctly instead of your lower leg popping off.
But it is all entirely up to you, that is the glory of horseback riding. That is just the ideal leg position for Dressage. (What I explained)
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