No matter when you decide to start tanking, there is going to be a learning curve. It's an entirely different role with different mechanics, and different mechanics targeted towards it. IOW, I don't really think progressing with a non-tank job first is going to alleviate any of the pressure and anxiety from tanking a duty your first time.

You can always look at a dungeon guide if you want a break down of boss mechanics if you are new, but definitely let your party now if it is your first time in a duty, or your first time tanking it if you've already completed it. This really goes a long way with keeping expectations at a reasonable level most of the time.

Qarn is the first dungeon with a high threat level (one shot mechanics), so try not to feel too bad. Not every dungeon after has this same threat level going forward, but yes, you are going to have to get used to dealing with multiple things at once. Final sting from bees requires them to prioritized, and the first boss has unavoidable doom PLUS bees xD. All players repeating Qarn should know to focus bees down so you don't get hit with final sting, but I would not rely on this. Place a [1] marker on bees, and try not pull more than one.

The first boss of Qarn will inflict everyone with doom when you see the room briefly turn orange/red. If you are dealing with this and bees at the same time, be sure to cleanse your doom first, and then focus down the bees. These bees have a much smaller HP pool than the trash bees and go down quickly. So long as they are not hanging around too long you should be ok. I know you might be concerned with moving the boss around too much, but being a tank does not mean you are nailed to the ground. Absolutely move if you have to and then reposition the boss as soon as you can. This boss does not have a frontal cleave so your party members should be safe so long as you have aggro on him.

The last boss is literally a tank and spank for you. The only thing you have to do is make sure he stays faced away from the rest of the party because he does have a hard hitting frontal cleave. Everything else is all on the DPS and healer, especially the adds. But they can only handle them correctly so long as you do not attack them.

Tanking, like anything else takes practice. The more you do it, the better you get at it. So if you want to be a tank main then switching to a DPS or healer is not going to give you the rounds you need to become a good one, and you should always play what you enjoy the most. Learning is part of the fun and I would enjoy this process even if you get discouraged from time to time.