Your current weaknesses are PW s (as they ignore FNP) and that you do not make full use of Fabius' potential. The enhanced CSMs would profit from a mark of Tzeench (against PW s) or Slaanesh. Mark of Khorne makes little sense since berzerks are better, same goes for mark of nurgle. The PW champion needs a melta bomb just in case.
Since plague marines are fearless squads of 5 do well, adding a third squad of 5 instead of two of 7 lets you choose more special weapons and makes it cheaper to sacrifice a squad as a roadblock if need be.
You need something solid to back them up, either more oblits (better organized in three squads of two so they can pick more targets) or a demon prince (wings recommended, rest is very optional).
Since you already have the plague marines, maybe also give this 500 points list a try.
Demon prince (without extras) + 3 squads of 5 plague marines (one with flamer)+ 1 spawn of chaos (at 500 pts a single one is actually useful)