  The elusive massive black hole (MBH) seeds stand to be revealed by the Laser
Space Antenna Interferometer through mergers. As an aftermath of galaxy
mergers, MBH coalescence is a vastly multi-scale process connected to galaxy
formation. We introduce the "Massive black hole Assembly in Galaxies Informed
by Cosmological Simulations" (MAGICS) suite, with galaxy/MBH properties and
orbits recovered from large-volume cosmological simulation ASTRID. The
simulations include subgrid star formation, supernovae feedback, and MBH
accretion/feedback. In this first suite, we extract fifteen representative
galaxy mergers with seed MBHs to examine their dynamics at an improved mass and
spatial resolution (by $\sim2000$ and $\sim20$) and follow MBH orbits down to
$\sim10\,\text{pc}$. We find that the seed MBH energy loss and orbital decay
are largely governed by global torques induced by the galaxy merger process on
scales resolvable by cosmological simulations. Specifically, pairs sink quickly
if their orbits shrink rapidly below $1\,\text{kpc}$ during the first
$\sim200\,\text{Myr}$ of pairing due to effective energy loss in major galaxy
mergers, whereas MBHs gaining energy in minor galaxy mergers with head-on
collisions are likely to stall. High initial eccentricities
($e_\text{init}>0.5$) and high stellar densities at kpc scales
($\rho_\text{star}>0.05\,M_\odot/\text{pc}^3$) also lead to most efficient
decays. $\sim50\%$ high-redshift seed MBH pairs experience consecutive galaxy
mergers and are more likely to stall at $\sim1\,\text{kpc}$. For a subset of
systems, we carry out N-Body re-simulations until binary formation and find
that some stalled systems merge at high-z when embedded in sufficient nuclear
star clusters.
