I’m assuming, given the fact she’s on medication for the itchiness, that things like fleas, mites, and bacterial or yeast infections have been ruled out.
If the itching is diet-related, then a simpler food, along with kefir, a fermented milk drink, might help. It’s a really strong, natural probiotic and great for promoting good gut health in both animals and humans. It can be a bit pricey to buy, but you can also make it yourself for no more than the cost of the milk once you get going, and it can help build resilience to food intolerances. It would be a long-term maintenance solution, rather than a quick fix, (and goes without saying that any changes to diet are something for a home owner to action, rather than a sitter).
However, there can be multiple causes for the itchiness, and it’s virtually impossible to pin it down to one culprit. Even though she has no broken or dry skin, it’s probably best to avoid bathing her with any products, or applying anything topically, as doing so could cause a bad reaction and make the problem worse. But if a swim in a clean, flowing river is an option, that could offer some relief. I’d also be looking at what detergent is used for washing her bedding, and whether switching to something hypoallergenic might help. Antihistamines, and plenty of Omega-3 Fatty Acids as part of her diet would help.
The scratching can also become something of a habit - it feels good to scratch when she’s itchy, but can also become an outlet for nervous energy if she’s stressed, or even just bored. But scratching leads to itching, leads to scratching and so on… You could distract her with toys, walks, games and reward-based learning - does she still scratch if her focus is elsewhere? Or if you want her to settle down and relax without scratching, try taking over the scratching for her, and gradually turn it into a stroke, and then just a hand resting on her.
Poor pooch. It’s no fun being itchy, so hopefully you can find a way to help her.