First and foremost, you are not "new to the role" passed level 50. By that point, you'll have access to nearly all your defensive CDs. Now I can see being a little apprehensive at 50 or even at the beginning of a dungeon but that first pull really should tell you how little mobs actually do. At the very least, you should be open to experimenting with larger pulls throughout the 50 dungeons. Tanxiety is just an excuse, especially at the higher levels. I'm sorry but saying you're not comfortable pulling two packs in a Shadowbringers dungeon means you aren't ready to tank above 70. Go back to the earlier dungeons and practice more. This is actually what irks me more than anything. A lack of willingness to try. Saying you're not comfortable yet is perfectly fine. But then don't jump into Hero's Gauntlet and get upset when the DPS and Healer want big pulls or even pull for you (they'll survive on their own. That's how pitiful most of the damage is). Either being open to trying to branch outside your comfort zone or stick to the 50-60 dungeons until you are.
Undergear and keybinds aren't a good excuse. The game all but gift wraps you gear on a silver platter. If you're far enough behind that six mobs is in any way threatening, you're purposely neglected to gear your character—which often means you're just being lazy. Likewise, you shouldn't be adjusting keybinds and macros in a dungeon. Go practice on a target dummy before queuing. That being said, by 50—which was the range I had been referring to—you should already know where things like Rampant, Vengeance are. You'll have had them for 42 and 12 levels, respectively.
Test pulls typically are two packs. A single pulls deals so little damage you could let the DPS tank instead and the healer would still barely notice save the occasional dungeon. Even if you're testing the outgoing damage, pulling three mobs just isn't worthwhile. As for being thin skinned. You're more likely to have players complain or outright pull for you if you single pull than if you attempt a mass pull, wipe and slow down from there.
I don't disagree players are afraid but the person I initially quoted implied tank was remotely complex when it isn't. That, in and of itself, is part of the problem. People making tanking out to be extremely difficult. Nevertheless, the reason it's under-populated is largely because DPS jobs are built to be more engaging.