I won't weigh in on rotation/optimization on MNK as I dont have experience as one. However, getting a Seiryu EX weapon at this point should be quite easy for you if you're willing to invest a little bit of time on the party finder. Groups are regularly doing clears and farms at this point and you should be able to get into some of those.

Also, now that the weekly tome cap was raised, you should be able to replace some 380 pieces with 390. Coupling that with Orbonne Monestary and/or doing all the Stormblood 24 mans you could get more than enough 390 gear as well as the items to upgrade the tomegear to 400.

While all of this won't contribute to the perfection of your job (which is what your interest is), it would help increasing your numbers a bit though.