Rework the whole cultural conversion and area of influence thing, because it can get crazy and makes late-game unappealing to some. Change it so it fills the available political space flexibly instead of creating its own infinitely, which could be influenced by

  • the systems and the "strength" of said systems (the visible influence of new systems could still grow gradually but would stop after a point),
  • battles won and lost in the region,
  • diplomatic agreements ("border agreement", to say dibs on yet unconquered minor factions or empty systems, secure important routes to move around fast or peacefully, etc.),
  • etc.