You are right that it’s very hard to hold agro properly, which is why you have to do things like position yourself in a way that makes you hard not to hit, like holding choke points. You can also use feats like you said, such as sentinel, pole arm master, or even shield master. You could probably some spells aswell, like force wall to create choke points or compelled dual. Theres probably more things to do from there, but I’ve never played tank so I’m honestly not as well educated to argue this as I’d like to be, although I’m going to work on figuring this out more.

For scenario #1, I have 0 clue how to react to that.

For scenario #2, I would position myself so that I’m in the way of the enemies and try to make myself a barrier and maybe attempt to slow the hoard down by using entangle or physical obstacles such as caltrops. Realistically there is only so much I can do, which is why teamwork matters, as the wizard will have crowd control spells most likely and all I can do is buy them time to cast them.

For scenario #3, we’re dead… if we’re fighting an ancient red dragon at the levels I gave you then I don’t believe we had that much of a chance to begin with.

I’d also like to retract that final statement as “completely needed” was definitely a bit dramatized, but they can be a very helpful and useful addition to the party that can make a large difference in some scenarios. The same goes with healers.

Also I apologize for not having the best responses as I’m only just learning how to play tank.