There is a nice table that shows, that Americium-241 has low power density, but it's half-life is 432 years. It also mentions that the real reason, why this is interesting, is because Am-241 is a byproduct and a pure isotope that can be used without refining.
It's not that simple. The energy per event varies by orders of magnitude depending on the specific transition (even within given particle decays, the energy of the emitted particle varies), also the number of atoms per volume or mass varies depending on material.
It has to first be separated from the rest of the actinide waste, which is very challenging, as they're chemically similar elements. Previously this had to be done with a centrifuge (separating by mass), but a lot of research effort has gone into developing more practical chemical refining methods.