On behalf of the BLM community, my apologies for your question going unanswered and mainly me ignoring your post and assuming it would be answered.
The INT+4 / MAB+2 route is the popular choice. HMP+3 is very nice and is the current best you can obtain for BLM head. INT+6 can be replaced with a Demon Helm but your inner Elf welcomes the +MP/Conserve MP. Magic Accuracy is nice but inferior to relic hat. If you have no plans on obtaining Sorcerer's Petasos, well that only makes it more difficult for you in endgame. You're going to have to ask yourself how much you want to gear this job. Do you want to be the best (have a lot gear options) or have the job ready for when a situation comes up (minimal options, somewhat effective). As for fast cast, that 2% isn't game breaking and FC overall is specific to a few special situations for BLM.
If I were you I'd settle with either the power option (INT+4/MAB+2) or the HMP option (make the hat part of your max MP set). If you go the HMP route you can use an augmented Demon Helm with +MP. You see that's the other thing. That Royal Redingote you got is starting to look pretty bland with all these recent awesome augments that have been happening. Looking further into the future SE might be (like that have recently) pushing for more Magic Crit Hit. Those critical hit percentages are going to start adding up and depending on your lucky streak, see very frequent procs.
I hope this helps you with your decision~
Gratz on the level up.