Pardon, I missed the bit previously about your dad being a farrier. Absolutely no slight on him to say so, but there are a lot of relatively recent developments in theory & practice regarding treating laminitis, that he may not be up with, so may be worth you - &/or him doing some further study into it. hoofrehab.com & also ecirhorse.com are 2 great sites to start with.
Horses are generally quite stoic. If your pasture is 'not hard terrain' and she is not being worked, yet you have seen that she is 'slow', then I'd say she's likely in a quite bad way. Generally 'founders' that are well managed aren't chronically sore just in a cushy paddock. Comfort is important for health & rehab, not just for... comfort. So horses who are sore even on soft footing are best with hoof boots with soft pads, until they get past that stage.
Re her diet, 'pellets' and 'hay/grass' doesn't give enough info I'm afraid, on whether her diet might be at fault. ecirhorse.com will give you info to better understand those factors too. There's another website, safergrass.org (I think it's org...) that is also good. It's possible it's the pasture/grass that is a problem.
Yeah, 'sweetfeed' is 'junk food' for horses, like giving them lollies for meals & is not good for them, so best avoided. But depending what's in the rest of her diet, it could be quite 'junky' too.
I'm quite concerned with your vet's view that "wasn't much else we could do for her as it seems to be something she's prone to keep doing." as this shows a serious lack of understanding about laminitis & it's factors - as said, lots of recent developments, so perhaps he's a good but just rather out of date vet... What tests did he do on her? There is a LOT you can do for your horse. Get her off too rich grass/hay, pad her feet, for starters...
Re biotin, this is one nutrient which has been studied in relation to hooves, and it's been found to help hooves grow faster. It is also a vitamin that horses tend to get enough of in their diet already if they have adequate green forage - grass, alfalfa, for eg. There is nothing, to my knowledge, to say that it could help laminitis or such.
Biotin is also but one nutrient, of many which are important, and also nutritional balance is important - it's not just about providing certain nutrients, but how they interact with eachother. You can't just take one ingredient & expect to 'make a cake', or just throw random quantities of correct ingredients together & make a good cake. So doing a diet analysis to work out what she IS getting, then finding a supplement which will fill ALL the gaps in her nutrition appropriately will be possible.
All that about cakes said... There is one particular nutrient that would likely benefit her & that is magnesium. Studies & trials on humans first, then on horses, have shown that when the diet is rich in sugar, that is one reason that magnesium levels are likely too low. And that magnesium supplementation can actually resensitise the body to insulin too. It has been used successfully to treat type 2 diabetics and insulin resistant / 'laminitis prone' horses. It has also been found that stress/pain cause a depletion of magnesium, so can be a 'vicious circle' once it's low.