I'm having similar issues. Just keep the horses trimmed short and do your mustang roll a bit more "severe". I've been keeping mine on a 2-3 week trim schedule, basically just touching up the mustang roll. This has helped them grow out and reduce the cracking.
Keep the toes cut/rasped to the level of the sole at the front, and keep the fronts rounded. In your first two photos, you can see a bit of a "square" toe developing. If you can round that off on the sides of the toe a little, that will help prevent the crack from spreading.
On the side, the quarters chipped out. That might be your horse's way of telling you that he needs those trimmed shorter than you were.
Don't put any topicals on his feet and try to give him a dry spot to stand if you can.
Anyhow, that's my non-professional opinion anyway
. I've been trimming my two for 2 years now and my mare goes through bouts of chipping and cracking during really wet weather. The above has worked for us!