In my experience, with quite a few horses with cracks over the years, the two primary causes are wrong diet and wrong stresses on the foot.
As an example of the latter, I've just had a horse operated on for kissing spines, and a long standing crack in a hind hoof has grown out. His incorrect movement was causing the hoof to crack.
Lots of cracks can be resolved by removing the shoes and allowing the foot to grow onto the balance that it wants. My first barefoot horse had an unstable crack in a hind hoof which grew out the moment the shoes came off.
A mineral balanced low sugar high fibre diet is a must.
im hoping you get some more replies to this one op.
I have the same problem with my horse...he is 23. I have had the shoes off for about two months now. the vet recommended biotin supplement and I have been applying kevin bacon hoof treatment for two months. I was told this could be down to his age, he is on ad lib hay with a scoop of speedibeet , a scoop of veteran mix and a scoop of molichaff. other than the hoof cracks he is in fantastic condition.
People are seeing good results with one of the two mineral balancers on the market with high magnesium and copper and no iron or manganese. Pro Hoof from eBay or forageplus.
Most UK analyses are coming back with iron and/or manganese overloads that cause copper deficiency which causes issues with insulin which causes poor hoof quality.