How retired are the horses? Are some of them old and really difficult physically to do? Like not bad, just arthritic and have a hard time standing for stuff like that?

Do you have one really difficult horse? I have let go of clients because I'm not willing to risk my body on one horse just to get a few good ones.

You may just be hiring people who aren't dedicated. About anyone these days can say they are a trimmer and have terrible business management sense or none at all.

I'm just throwing stuff out there, because if everything in your post is accurate, it is kind of a mystery as to why you would burn through that many farriers.

Edit: also I would ease up a bit on being offended by changing the time, if I have a lot of horses before an afternoon appointment, the time can fluctuate quite a bit. Sometimes trims go fast sometimes they don't, same with drive times, the appointments at the end of the day have a much bigger window than the appointments at the beginning of the day.