On a new/fast evolving product I prefer to have a solid suite of integration/e2e tests and a lighter unit test suite near the edge of a server with no mocking of deps (e.g. if it uses a db, spin up a local instance). I would also test something that is non-trivial to understand or a critical dependency in the system e.g. a rules engine.
The reason being is - the code is in so much flux that the internal interfaces change constantly. In agile new requirements come along and you end up chucking a lot of the old code out of the window and wasting time.
As you move towards completion of the project and the internal interfaces shore up then increase the tests. So when it’s in maintenance mode someone else can easily make modifications and has documentation on maybe why something works in a certain way