I have all of my Disciples of War/Magic to level 50, but as much as I'd like to start on ACN, I would never ever
eveeerrrr not start in Gridania. I love it way too much and am much too attached to it to start anywhere else! Plus, like you, it suits my character and her story to be starting in Gridania again. I can't imagine her anywhere else. It had such a cool story in 1.0 and I always felt really welcome there because Miounne was always contacting my character on the linkpearl during the starting story and always giving me advice and stuff on where to go/what to do! It always made me feel really involved and it was such a cool experience the first time I played! Compared to the other two city-states in 1.0's starting stories, I think I had the most fun starting in Gridania. And even though I've played through the new story fifty times during these betas, it gets more interesting each time, from finally showing us the full cutscenes in Phase 3 to voice acting in Phase 4! Also because I'm a tried and true Adder and Kan-E-Senna's my homegirl, so I wouldn't ditch her even if someone paid me a million gil.
I'd probably pick CNJ to start since WHM's like my secondary job next to THM and then switch back to THM as soon as I can since the experience from those quests isn't really going anywhere anyways. I'll most likely end up running through the story quests that keep you in Gridania and then maybe see if I can run to Limsa after the level 15 quest, get ACN, make a mad dash out of Limsa (because I have no doubt it's gonna be like so laggy and crowded with everyone and their mom starting on ACN.), and then level ACN to 15 or whatever with the non-story quests and maybe some FATES I missed in Gridania, then continue on with the story so I don't need to be level-synced for everything.
If you really want to level ACN you could probably do the same and try to rush to 15 after you're able to get to Limsa; there are a BUNCH of sidequests to level you up no matter where you are.