& now after reading other posts...
Firstly yes HLG, zinc is one of the vital nutrients often lacking, which is associated strongly with hoof health. As is copper, magnesium, etc, etc. And then over supply of iron, manganese, potassium, etc, can also be a problem. So OP it depends what she is getting already in her diet as to what she may need supped. Biotin is but one very well marketed ingredient that may or may not be necessary. If she's getting adequate green pick, then she may well be getting enough biotin without supplementing. So first & foremost, you need to do a diet/pasture analysis, before you can know what supps may be needed.
She was getting Triple Crown senior, which is what she’s eaten pretty much her whole life and like I said her feet were fine.
Why did you feel the need to feed that to her, if she were young and healthy? Was she having trouble keeping condition? Was she in hard work? Was she not getting enough grazing, hay, other feed? And what was the rest of her diet?
Yes, she was likely 'fine' just as, for eg, most people are 'fine' if they live on junk food for some time. Things like type 2 diabetes don't tend to rear their heads until someone has been eating badly for a long time. **Not at all that I'm saying this is a bad feed per se, but just because 'she was fine' doesn't mean to say diet has been good.
I just in the past month switched her to Strucomix Senior because I wanted to feed something lower sugar, but it’s too recent a feed change to see if it’ll improve her hooves. I’m feeding her according to the guidelines on the bag and it is a high-quality feed.
TC Senior IS low sugar, grain free. It's only 11.something% NSC and is designed to be an appropriate food for horses with metabolic issues, and even as an entire ration for old toothless horses. While a quick look at that Strucomix shows that it is a high grain content feed(even including corn!!) so, while their product sheet doesn't mention NSC/sugar/starch, it looks like a 'high octane' feed, I highly doubt that it's at all low
(let alone lower NSC/sugar than TC). (just saw Rambo's comment about that feed) Again, I wonder, why have you chosen those particular feeds for her & what is the rest of her diet?
Don't forget, while 'feeding according to the bag' may well be needed, it depends on the condition of the horse, what else they're getting as to how much they may need, and of course, feed co's also want you to feed/buy as much as possible, so guidelines are on the upper end of the scale, very often too much for most average horses.
I’m expecting the Ferrier to be out this week to look at another horse so any advice I get here I’ll run by him when I see him.
Horses should generally have their feet attended more frequently than the norm of 6-8 weekly. Generally 4-5 weekly is good. Especially as the horse has had serious & ongoing lameness issues, even has trouble on hard ground in padded shoes, I absolutely wouldn't advise you leave her longer than about 4 weekly max between FARRIER visits.
As usual, I agree with what Walkin & Rambo have told you.