In all of the dorsal photos, he appears to be slightly laterally high. That would tip him onto his medial side more, which could explain why the medial side breaks off. At this stage, we should be aiming to get the heel heights to just a smidge above the sole plane height. Anywhere the walls are not well connected, there should be no wall length beyond the sole plane - leaving it even a few millimeters longer will only cause it to break off as it isn't structurally sound.  When measuring collateral groove depths, make sure to find the deepest point  - which may/may not be even with the location of the heels. Place your measurement tool down into the groove  all the way to the bottom; hold it upright, not tilted; take the measurement based on the height of the sole (no wall beyond sole) at the perimeter of the hoof. Same idea for the depth near the apex of the frog. See figure 8 here:

https://www.hoofrehab.com/HeelHeight.html

 

His heels continue to be somewhat flared, with a cavernous distance between the bars and the frog. That needs to tighten up so that his heels are actually under his leg, supporting his bony column, rather than splayed out on either side of it. His frogs need to develop more fully, which means he needs to be landing solidly heel-first and engaging them with the ground (or engaging them in his boots with the pads).  Need to get those walls in the heels tightened up, out of weight bearing and rocker the backs of the buttresses. Figure 2,3,4 here show what's needed:

https://www.hoofrehab.com/HeelHeight.html

Cavallo now has some thicker pads for their boots which I would prefer over their other, thin gel pads as they provide more cushion and have more of a supportive function. They are very similar to the Easycare Comfort pads, which are a lot less expensive and have 3 different densities.