  It is well established that supermassive black hole (SMBH) feedback is
crucial for regulating the evolution of massive, if not all, galaxies. However,
modelling the interplay between SMBHs and their host galaxies is challenging
due to the vast dynamic range. Previous simulations have utilized simple
subgrid models for SMBH accretion, while recent advancements track the
properties of the unresolved accretion disc, usually based on the thin
$\alpha$-disc model. However, this neglects accretion in the radiatively
inefficient regime, expected to occur through a thick disc for a significant
portion of an SMBH's lifetime. To address this, we present a novel 'unified'
accretion disc model for SMBHs, harnessing results from the analytical
advection-dominated inflow-outflow solution (ADIOS) model and state-of-the-art
GR(R)MHD simulations. Going from low to high Eddington ratios, our model
transitions from an ADIOS flow to a thin $\alpha$-disc via a truncated disc,
incorporating self-consistently SMBH spin evolution due to Lense-Thirring
precession. Utilizing the moving mesh code AREPO, we perform simulations of
single and binary SMBHs within gaseous discs to validate our model and assess
its impact. The disc state significantly affects observable luminosities, and
we predict markedly different electromagnetic counterparts in SMBH binaries.
Crucially, the assumed disc model shapes SMBH spin magnitudes and orientations,
parameters that gravitational wave observatories like LISA and IPTA are poised
to constrain. Our simulations emphasize the importance of accurately modelling
SMBH accretion discs and spin evolution, as they modulate the available
accretion power, profoundly shaping the interaction between SMBHs and their
host galaxies.
