Only two reasons to do this:

1. If already fully committed to resigning, to extract some benefit for a few weeks in the interim. Say a conference junket, or deferral of some mandatory training.

2. If part of a powerful group, to extract a better deal from management. I know of some engineers at FAANG that blocked plans for downsizing of cubicles this way.

Yes-because it is important to keep a clean distance from finished work.

Work as if you will resign on completion; it will enforce a sense of responsibility to document business processes, code, contacts-you name it.

And where toxic processes or customers do exist, it is important to remember that you have the power to walk away.

Nope

I only tell them when I actually am

Until then? Why give a reason for them to get rid of me?

Once I asked my manager if he would be ok with me putting him down as reference. He panicked a little bit. Then I got a bumper bonus.

yes

No. Bad idea.