Copper Sulfate and Chlorine dioxide are your friends. Aka Kopertox and White Lightening. I would suggest washing your horse’s feet in the white lightening/vinegar bath (letting them dry) and then putting the kopertox on.
I would also suggest running all this by your vet because you may want to treat with oral antibiotics if it’s deep enough. There is NOTHING like trying to chase after thrush that has already gotten beyond the scope of topical treatment.
I would also suggest keeping the feet DRY. Possibly restricting turnout to during the day when the grass is dry (after the morning dew dries and before the dew starts to gather in the evening).
If you are able to put this horse in boots with diapers on to keep the feet dry and clean, that would be ideal. I’m a big fan of the Cavallo Simple Boots and the Old Mac G2’s.