Those animals (e.g. tortoises, lobsters) with longer lives combined with less cancer have significantly lower metabolisms and a far less ... intense ... lifestyle.

Higher cellular respiration introduces higher chances for mutation / inflammation / cell damage. Likewise, high rates of cellular reproduction do the same. Every incidence of that is a roll of the dice for cancer. Extend the lifetime, and you will for sure get cancer, because you already have a very high chance of it.

Evolution in our species has arrived at a set of trade-offs. We don't understand them all yet.

Bats have surprisingly long lifespans for their very intense lifestyles

It is naive to think that human biology has already found the globally optimal trade off between longevity, cancer and metabolism.

Sure we don't understand it all yet, but there's no harm in trying to.