There are persistent rumors that IV-vitamin C in large doses -- administered by a real doctor in a clinical setting (watch the pH and electolyte balance, please!) can make a difference.
CAR-T cells look to be the real thing in many cases, but costs are currently very high (~ $500,000).
I think keto might be called for with cancer, yes. No need for no carb. The gut bugs play an important part of our immune system (like most of it) so we must keep them in good standing and fed. If the carbs are natural starches (veg and tubers, seeds, beans) you can eat 50-75 g and still be in keto. I don’t aim for keto but there have been times when I couldn’t escape it without really trying.
Protein needs to fit activity and growth (incl cell growth) but I know what you mean about too much becoming sugar. Healthy fat is the key.