The bottoms of my justins are pretty slick, maybe that's the issue, they work well with the cheese graters. The bottom of my new saddles leathers/fenders are 1.5" so don't really work with my Widetrax chease graters that need a 1" leather. The problems I had with the endurance stirrups were when on an Aussi saddle which does by design put you in a bit of a chair. I adjusted the leathers up and down a notch, to see if that helped. To long and I would loose them at a canter on occasion. But was comfy on my knees. I would guess the stirrups were about like yours , the bottom hitting just below the ball shaped ankle bone. Then down a notch and back up. On my aussies Id adjust while riding.
Next ride I am going to use the stirrups that came with the saddle, they are about an inch taller so probably give me the length I need. Going to ride in some Danner Light hikers see how that works for me, they have pretty agressive treads. If they work out will order some cages to put on the stirrups I have, maybe shorten them some to keep my foot where I want it. OR I might take the pad off the endurance stirrup and rivet an English traction piece to it. OR change it to Wintec webbers.