I am definitely obsessed with that topic, LOL
I basically am a big believer in what you hinted at namely: Based on what I have read in the scientific literature I feel there is little doubt that diet can influence cancers of all kinds. (Plus I have a good friend who beat a late stage prostate cancer and so I'm following his footsteps.)
Personally I was mostly vegan before my diagnosis as my wife and daughter have been fully vegan for quite some time so switching to a 100% vegan diet was easy as a starting point. Next, however, I have a personal guess that I helped my cancer along by drinking too much red wine (as in 500 ml to 750 ml almost every day for 20 years) and now I feel that depriving the cancer of all it's possible favorite foods then maybe, just maybe a "healthy" diet in combination with radiation and chemo will at the very least slow and the very best eliminate the cancer growing inside my body. As such I am trying to reduce or even eliminate the list of potential culprits that the scientific literature tells us MIGHT be aiding and abetting our cancer.
So . . . I have eliminated meat (including chicken), dairy, and alcohol totally and aside from a few cheats I try to stay away from gluten and high glycemic foods like white rice. It's really not that hard so long as I shop for the right foods and don't buy the other stuff. My dietary goal is to stick to fruits, vegetables, legumes, nuts, and complex starches like oatmeal, brown rice, quinoa, etc. Of course, I "cheat" with a granola bar, or Impossible Burger, or really cheat at a Thanksgiving dinner, but in general I try to stick to a gluten free vegan diet. I also use calorie restriction off and on as well. Not intentionally, but just as a function of skipping meals due to a busy schedule including treatments etc. I think the scientific literature on fasting or other forms of calorie restriction are pretty clear as regards inhibiting cancer growth.
I should mention that I also try to hit 300 mins a week of exercise, but by exercise I'm talking fast walking, squats, planks, and light weight lifting with high reps and 5 sets of things like curls, lat pulls, tricep bends, etc.
Now will this make a difference? I have no idea, but it makes me feel good thinking that I am trying AND it's generally considered healthier overall. So far the hardest part has been reducing sugary foods like putting honey in my oatmeal or eating a granola bar loaded with peanut butter and sugar.