I'm personally not convinced it really needs much if anything at all. Build it as a gunboat with attached interceptors, or build it as a carrier with guns, but asking for both is like asking for a wonder weapon that does it all.
To quote a current chide: "Skill issue."
But that isn't a helpful thing to say. Honestly I find even my regular legions to be decent defensible carrier ships. No astrals, but nor should they be, and no onslaughts but again that's not their place.
I'll say that yes the Legion is no dominant capital ship, but if you can't make them work in your fleets then that isn't the ships fault.
Give it a 10-20 op boost and reassess. No need for anything dramatic.