Greetings to you, and welcome back to FFXIV.
Server Transfers are sadly the only way to get on Balmung or waiting for that right moment before/after maintenance. Unless they changed something, you should be able to transfer in still, or at least this was true prior to 3.1 when I transferred my alts to Balmung from another server. It never hurts to check if it's possible and it doesn't cost anything, but time, to see if you can. Unless of course you can and then go through with it, but that's a different story. lol.
The major hub is the Quicksand, the other adventurer guilds in Limsa and Gridania do have people in them from time to time but they're not as active as the Quicksand. The even more rare place to see RP is The Forgotten Knight, a bar in Ishgard. Other then that, you'll find people RP'ing just about everywhere. Even though it's not always in public view. (I'm looking at you Hermit's Hovel.)
Hope this helps answer your questions and once again, welcome back. :moogle: