Let's deal with the fallacies first.
"nature, which has already provided everything we need for our survival."
True and false. We have changed the natural environment around us such that we can survive in it.
https://images.takeshape.io/fd194db7-7b25-4b5a-8cc7-da7f31fab475/dev/71d1627b-ab60-4112-833c-fee00220cb5f/Maize-teosinte.jpg?auto=compress%2CformatThis link is an image of how we have changed corn/maize over time. There is nothing "natural" about the systematic selective breeding i.e. genetic engineering we have done to it. Even if once upon a time nature was trying to help us out (which i dispute) theres no reason the genetic traits it had that helped us should still exist.
"Natural remedies are the best remedies because humans and their natural food sources are in symbioses."
Could you show how we are in symbiosis with our environment? It looks to me like we've completely and utterly dominated it with machine, pesticide and selective breeding. Also what about something being natural makes it good? Scorpion stings and arsnic are both "natural". I'll take antibiotics over those anyday when im ill.
"Our fruits and vegetables depend on animals to spread their seeds through their faeces",
False: Our fruits rely on us to spread them plant them use pesticide on them and so on, all natural defences they had to disease have been weeded out excuse the pun, in favour of higher yields higher edible content. For example, Did you know bananas are sterile? And going extinct? Because of us?
"and will never deliberately hurt us." Fruits are not sentient. Lots of fruits are poisonous, there are lots of berries we can't eat, we just don't cultivate them because why would we?
"In fact, they have evolved to benefit us, as we benefit them, and make sure to feed us with an assortment of vitamins and nutrition as an incentive."
we have selectively bred them to benefit us, but we have breed them for high yields, not vitamin content. Not that vitamins cure cancer
"We also have shared enemies with the plants, such as fungus, bacteria, and viruses. Plants will also get cancer, just like we do. It stands to reason, therefore, that anything a plant makes to repel those things will benefit us as well."
This is probably the only sound thing you've said. And its true we do extract many pharmaceutical ingredients from plants.
Cancer treatments, however, are much harder to find than an antibacterial agent. Because cancer is our own body malfunctioning rather than a foreign invader. Cancer is what happens when our own system fails to regulate itself. Unfortunately, our body is also what we call a homeostatic mechanism. It keeps itself highly regulated. And since it sees cancer as part of itself(this is why your immune system isn't killing cancer) it also protects cancer. So anything you put in that tries to change this state of balance your body will simply adjust to.
I get that cancer treatment is fucking expensive, and often feels hopeless, but thats not a good reason to not see a real doctor. On the upside 2/3rds of people with cancer survive it with modern medicine. I doubt your grocery list will get that high a success rate. And an anecdotal handful of people who spontaneously went in remission is as much evidence as me calling heads and it landing heads 10 times in a row makes me psychic.