Second, the amount of material is not at all mentioned in your definition. One half of one gram of material, is not the same as the many kilograms of spent fuels from a working reactor, or the 2000 metric tons of spent fuel generated now in the USA each year. So I ridicule that in an unexpected way, with another extreme example, comparing something tiny, to something massively large that everyone knows.
There is some false confidence in my opinion, about "know the facts" of radiactive materials.. when bio-accumulation in the food chain, uncontained dispersion, poisoning fresh or salt waters where life exists, and human exposure over time, and repeated exposure, are serious things.
When someone says "(a substance) has a (very large) half-life" it is typically an attempt to make the substance sound more dangerous, when in general a longer half-life makes the material less dangerous, not more.
While there are certainly other nuances involved such as what kind of radiation is emitted (alpha, beta, gamma, neutron), and what the immediate fission products are, if all other factors are equal, the longer the half life, the less radioactive the material is. That's literally the definition.
The really nasty isotopes are ones like Iodine-131 (half-life 8 days, has an affinity for the human thyroid gland) and Strontium-90 (half-life 28.79 years, has an affinity for human bones). While saying that something has a "half-life of forty million billion trillion years" may sound scary-impressive, what it actually means is "this stuff is only very mildly radioactive).