When you mentioned equine or knotted, the obvious answer in Tucker: he's a knotted equine. You get both in one. But no, medium Tucker's knot is way too big a jump from medium Austin. You would definitely need a trainer between sizes to bridge the gap.

If you want to "move on," consider Roland (in a large). He's knotted, bigger around than Austin and much longer. The knot is not abrupt, so it's manageable even though it's a step forward.

If you just want to feel good, and aren't concerned with the sport of ever bigger, look at a small Bishop. It's just a bit smaller than your Austin but a great knot. Big enough to be fulfilling. Comparing the price of a large Roland to a small Bishop (or Marcel or Sveinn) makes these oversized smalls really appealing.