100% coverage is a start

How much “coverage” should your test suite provide?

100%.

But, I have worked at places with a 100% metric and they still introduced bugs and was super slow.

Yeah, 100% is a start. The tests also have to be good.

That sounds nice but Bob Dev is writing meaningless tests.

Software engineering is a team sport. If Bob isn’t a team player then some meaningless tests are probably the least of your problems with Bob.

Well, how do we get to a place of 100% and good tests?

There is not going to be a silver bullet. A good place to start is that you will need to provide a work place where people feel encouraged to practice the skills involved in writing tests. The biggest obstacle to a good test suite is not having the permission and support to practice.