  Parsec-scale separation supermassive black hole binaries in the centre of
gas-rich galaxy merger remnants could be surrounded by massive circumbinary
discs (CBDs). Black hole mass and spin evolution during the gas-rich binary
inspiral are crucial in determining the direction and power of relativistic
jets that radio observations with LOFAR and SKAO will probe, and for predicting
gravitational wave (GW) emission that IPTA and LISA will measure. We present 3D
hydrodynamic simulations capturing gas-rich, self-gravitating CBDs around a
$2\times 10^6$M$_{\odot}$ supermassive black hole binary, that probe different
mass ratios, eccentricities and inclinations. We employ a sub-grid
Shakura-Sunyaev accretion disc to self-consistently model black hole mass and
spin evolution together with super-Lagrangian refinement techniques to resolve
gas flows, streams and mini-discs within the cavity, which play a fundamental
role in torquing and feeding the binary. We find that higher mass ratio and
eccentric binaries result in larger cavities, while retrograde binaries result
in smaller cavities. All of the simulated binaries are expected to shrink with
net gravitational torques being negative. Unlike previous simulations, we do
not find preferential accretion onto the secondary black hole. This implies
smaller chirp masses at coalescence and hence a weaker GW background.
Critically this means that spin-alignment is faster than the binary inspiral
timescale even for low mass ratios. However, we find that mini-disc and hence
spin alignment is not guaranteed in initially misaligned systems, potentially
leading to a significant fraction of recoiled remnants displaced from their
host galaxies if chaotic accretion is the dominant feeding channel.
