Before the movie came out I assumed the Sith fleet was just stockpiled ships from the old Empire. Someone pointed out the bridges match the ISD-1s, which we never saw after A New Hope. Basically it seemed they were phased out for the ISD-2s that we see in ESB and ROTJ. So my thought was Palpatine sent them all there before the Empire fell as a contingency, and the Sith cult modified them with the laser canon.

Apparently the Visual Dictionary says they’re a brand new class, but I feel like it makes so much more sense if they were just modified ISDs.

And if you stuck with that idea, then basically all the First Order needed to do was boost their numbers to man this new/old fleet.