You seem to be doing pretty good with heel-toe. I experimented with heel-toe for a while several years back, even going so far as to actually get longboards for that very reason, and I did not end up liking the technique. First (as you said) if you have large feet it is very difficult to do. I can tell you that in your case, with a size 15 shoe, longboards won't necessarily solve your problem. I have about a size 12 foot and played in my socks, and even that extra added couple inches did not give me enough room to do the technique. I had to either move the ball of my foot way up toward the end, or hit the part where the heel joint connected to the pedal.
The other thing is that I found the pedals difficult to control with heel-toe.
So far I have been using a whipping toe-toe motion (where the leg comes down and the ankle rocks back and forth, but the toe makes both strokes - very similar to
this technique) on a standard pedal and find this works well for executing fast double strokes. I get a lot more control over the speed and intensity of the strokes. But I don't know how well that kind of technique translates into speed double bass, to me it seems it will wear your ankles out real quick.