I'm just curious, do you know what the opinions about this stuff are from people that work in these fields, or that have dedicated their lives to it?
Check out this paper from the Lander lab: https://elifesciences.org/articles/61026
It’s a bit jargon heavy but it’s a nice case study in how tumor growth is controlled through all the same mechanisms that normal tissue growth uses. Even cells with an outright cancerous gene mutation are basically still just doing normal growth and development.
In practice, though, some species are way less prone to cancer than others. Orders of magnitude of a difference, even in mammals. Bats, notoriously. Or naked mole rats. On the other hand, mice get cancer fairly reliably.
Which means that there are biologically realistic way how to keep the danger at bay, and they seem to involve the immune system.
Might be that cancer hits after creating offspring.