If the fih had ich that bad, yeah then it's time to say "good night".
Garlic is not a 'folk' remedy. It's not even meant as a remedy. It's not suppose to kill the parasite, but protects the animal from more parasites from latching on. It gives the animal a bad smell and a bad taste. Just as how garlic works for dogs and cats and even us humans with fleas and mosquitos. Doesn't kill 'em, but they find lunch elsewhere if they can...lol.
Having amphibians makes treatment difficult anyway. If you don't have any fish, the ich will die off from starvation eventually. If you do have any fish at all, then you need something or else the ich will continue in the system. An extra coating of slime on the fish may make it difficult for the ich to breathe and may allow it to fall off the fish. The garlic would keep the parasite off the fish until it starves. That would be the goal. Not saying it's 100% but worth a try. If it doesn't work, then there aren't any chemicals to worry about killing anything, especially the amphibians. They absorb everything through their skin.