but the particles are infinitely working to expand, as futile as the efforts may be

True, as long as the temperature is non zero kelvin.

Even at equilibrium, the particles are still in motion at an atomic level.

Same as the first answer. However, conceptualizing the gas molecules/atoms as darting about (aka billiard balls) is only one useful framework. It is not necessarily correct, or consistent with everything we observe. Gas largely behaves as a fluid, and is an alternative useful conceptual framework. Of course, in either conceptualization - constant motion of some sort remains.