both of the claims "1-2 pounds of watermelon/day kills my desire for processed desserts" and "This leads to weight loss, although maybe that also requires potatoes?" seem to me like they can be explained through the lens of volume eating. I am curious what you think of this theory of mine since you clearly have thought about it for much longer. In case you aren't familiar with the concept of volume eating, there was a  done where they gave people 240 calories of a food to people and then measured how hungry...
Epistemic status: just speculation loosely based on my understanding of how my brain works and assuming other peoples brains work similarly.
I think the problem might be that there are too many mysterious diseases. As an analogy: I have a desire to learn a second language, but often when I start trying to learn a language I will be distracted by other languages I could learn, I'll study one language for a few months and then abandon it for a different language, and then abandon that language for another, until I look back and barely remember any words from ...
I can only remember times I've heard "Better safe than sorry" used to encourage taking an action that had a high chance of being a small waste of time or effort, with a small chance of minimizing the consequences of an unlikely event that would make it worth it, such as taking an umbrella in case it rains or seeing a doctor in case there is something wrong. I can similarly only remember "Nothing ventured, nothing gained" used as a way to encourage taking actions specifically usually also ones that have a low chance of a high reward, and a high chance of a small loss. These do not seem like opposites to me.