That depends on whether you accept the risk of having to find another line of work. Depending on the place, you can do almost anything you want even at PhD level. I've been on a chain of grants, doing quite freely what I want, for over a decade. But I accept it may break at any point and then I'll just go do something else.

After PhD there will be nobody telling, and often not even caring, what to do. But that may mean that you don't get your PhD or you don't get another grant to live on. If you get a tenure it almost literally means that you can't be fired even if you do nothing at all. What is surprising is that almost all with tenure keep running the rat race even though they don't really get anything at least material out of it.

(Nitpick: I think serious research is the fun one. The one churned to get another grant is neither serious nor fun.)