As a fellow altaholic, I can sympathise with your plight.
I have 5 characters. Tiergan wound up being my main in the end, but I also have Lurial at level 50 due to my inability to decide on which one would be my main character. <_<;a I gave Tiergan all the melee classses while Lurial got all the caster classes.
As for my other alts - I got them to level 15, which is the level where you get your airship pass and are able to freely move about between the three cities. This enables me to use them exclusively as RP alts and I can level them up at my leisure after doing all the PVE stuff on Tiergan/Lurial.