Time-resolved thermoplasmonics is emerging as the go-to technique for nanoscale thermal metrology. In this context, connecting the ultrafast optical response of nanoobjects to the correct thermal pathways is of paramount importance. We developed full thermo-optical models relating transient spectroscopy measurements, performed on metal nanoobjects in dielectric environments, to the overall system thermal dynamics. The models are applicable to small spherical nanoparticles embedded in a homogeneous matrix, following an analytical approach, and are expanded to include the cases of arbitrarily complex geometries and sizes relying on the finite-element method. These approaches are then exploited to rationalize several observations made in the context of previous time-resolved thermo-optical studies at the nanoscale. The present tools open the path for accurate retrieval of thermal parameters, notably the Kapitza resistance and the local environment thermal conductivity, from experiments. They also allow identifying the optimal parameters for selectively probing thermal dynamics of either a nanoobject or its nanoscale environment.