  After more than five years of development, we present a new version of Dark
Sage, a semi-analytic model (SAM) of galaxy formation that breaks the mould for
models of its kind. Included among the major changes is an overhauled treatment
of stellar feedback that is derived from energy conservation, operates on local
scales, affects gas gradually over time rather than instantaneously, and
predicts a mass-loading factor for every galaxy. Building on the model's
resolved angular-momentum structure of galaxies, we now consider the heating of
stellar discs, delivering predictions for disc structure both radially and
vertically. We add a further dimension to stellar discs by tracking the
distribution of stellar ages in each annulus. Each annulus--age bin has its own
velocity dispersion and metallicity evolved in the model. This allows Dark Sage
to make structural predictions for galaxies that previously only hydrodynamic
simulations could. We present the model as run on the merger trees of the
highest-resolution gravity-only simulation of the MillenniumTNG suite. Despite
its additional complexity relative to other SAMs, Dark Sage only has three free
parameters, the least of any SAM, which we calibrate exclusively against the
cosmic star formation history and the $z=0$ stellar and HI mass functions using
a particle-swarm optimisation method. The Dark Sage codebase, written in C and
Python, is publicly available at https://github.com/arhstevens/DarkSage
