In "Sprint Planning", the team is supposed to come up with stories, break those down into tasks, estimate those tasks as a team, then let devs choose what tasks they want to work on based on the stories pulled into that particular sprint.

Instead, our manager creates the stories. He assigns the stories to each developer and then has that developer announce his theoretical tasks (without any research on feature's or project's requirements!) in front of the entire team. So, when I say, "I think it will take me 6 hours to implement this feature", he says, "6 hours? I think it will take 3." and then types the estimate as 3. I have so much rage when that happens. Then we continue to sit in the room for 2.5 hours where we go through this long data entry mess of him typing out tasks and second guessing estimates. There is no team deliberation or collaboration, its whatever the manager says.

While there are many issues I take with this approach, my pet peeve would be the second guessing of the estimates. It would make sense for teams members to second guess estimates as long as they are the same teammates who have the ability and possibility to take on the tasks themselves.

But I disagree with a manager seconding guessing an implementation feature that "I" definitely have to do alone, and they do not possess the immediate knowledge to implement it themselves.