If yor worried about caustic, stay away from most commercial thrush/WLD treatments then. But seriously I don't think you have to worry if its not on live tissue. The stronger the better, esp if you need to compromise & stay shod. 'Milder' treatments such as dilute bleach, ACV etc will work sometimes, depending on the type of bugs causing the infection.
Re tender feet, if hooves are unhealthy I think they're generally better unshod, but you may need to pad/boot if he's sore. I'm also betting that there are mechanical issues which are stretching the laminae & allowing infection.
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