The same result is obtained, for instance, using the Nakajima-Zwanzig projection technique instead. Moreover, we are also flexible in the choice of the expansion reference, allowing us to explore another, renormalized perturbation theory, to extend the potential usefulness of our results in regimes of strong coupling. This more powerful expansion was initially developed for K as first stage of a continuous RG-flow method for open quantum systems [5,47,48] to deal with strong dissipative coupling and non-equilibrium. It was later studied on its own merits [46,[49][50][51] and revealed powerful exact relations [50,52]. However, applications to the transient time-evolution of interacting systems analyzed in the present work were not yet explored.