OP, you are placing a heavy burden on yourself choosing, or trying to choose not to group with players with the role you enjoy most. Of course, this is your choice but I hope you acknowledge the amount of extra effort you will have to put in to just keep avoiding tanking for parties. I bring this up because you still want to progress the MSQ as a tank. Yes, in ShB you will have the luxury of the trusts to get you through the MSQ dungeons, but as has been pointed out to you, the trials will prove to be a significant roadblock. You will essentially have to keep a minimum of two jobs leveled throughout the MSQ: your tank, along with another you feel comfortable using in the trials to come.
Despite your tanking experience in other mmos, you still have to tank in FFXIV for people to accept you tank in FFXIV. A lot of what you have learned from other mmos will transfer over such as keeping mobs turned away from group, use damage mitigation abilities, have good gear etc. but you still have to get your feet wet in order to get good at it. The outer world does not, and likely never will present to you any kind of threat that is significant enough to increase your skill. S/A rank mobs don't count as they are not meant to be soloed, and all roles tank by default when they solo. IOW, you're not learning how to tank. This is done in dungeons.
Tanks face their greatest threat angering all the hornets in the nest. Otherwise known as trash pulling in dungeons. Bosses and trials are typically spank fests for tanks as the majority of the mechanics target the DPS and healers. Your inflated HP and high defense also gives you the privilege of surviving mechanics that would one shot you as a DPS or healer. If you go into upcoming trials as a DPS, the fights are actually going to be harder for you to not only perform, but also survive so I would keep that in mind as well especially if you are not wanting to be a burden to your group.
The way to not be a burden to your group is to speak up to let them know to keep their expectations at a reasonable level. If you are new to tanking a dungeon, just say so. 99.8% of the time no one is going to give you grief over it, and the .02% someone does you can just leave. Nothing is forcing you to group with players you don't want to group with.
Something else that is ironic and might be a tad bit comforting is that every single bad tank I've had was actually really good at it, and it was their pompous 'holier than thou' attitudes that made me not want to play with them. Talking down on other players, threatening to leave, giving unsolicited advice; it's things like that that will make you a burdensome tank no one wants to play with. Causing a wipe because you pulled too much? They are actually MUCH more willing to deal with that, my friend.
Get out there cowboy!
