Pictures would help a lot. Thrush is a general term sometimes that people use to describe bacterial or fungal infections in the hoof, they are very common. Picking her out daily is the best thing in the world, it does make a difference. Being deep, she could have long heels which will contribute to the infection, does she have a crack going up her heel bulbs? My mare had deep collateral grooves along the frog from infection when we brought her home. It made her severly lame so every day I carefully scrubbed her hooves after picking with warm water and blue Dawn on a mat. Dried then soaked her in White Lightening which is the best stuff next to Cleantrax to get deep up into everything and kill out any infection. You can google how people use it, many people give their tips that the company doesn't have on how to use it best to be most effective. It is more widely available now. Then daily we picked and then with a syringe put a dab of Pete Ramey's inexpensive hoof goo in the deep areas and cracks of the frog, grooves, and bulb. Using generic Wally World triple antibiotic cream 2 oz mixed with antifungal foot cream 2oz. It works amazing. But if she is long it won't clear up but will help her anyway. Bleach is way too toxic especially at 50% (just my humble opinion) it is caustic to the tissue. So it destroys any tissue that is healthy as it destroy's any bacteria or fungus.
Any horse can have thrush or fungus no matter how clean you keep them. It's in the manure, manure dust, dirt etc.