This will sound dumb, but what if Starkiller Base isn’t a planet-killer, but an ancient Sith bioweapon to resurrect the dead?

The First Order’s plan in TFA is to bring Palpatine back to life. This explains how he comes back in TROS and avoids “yet another death star”. Although it sounds dumb in concept considering what we have to work with I think it’d tie things very smoothly.

Alternatively, the FO could be destroying the Republic as a mass-sacrifice to enable Palpatine’s return. I remember something like this in SWTOR.