It's not really about productivity. It's about creating a sense of control over your life. The reason there is an industry around this is because the need is never sated, and the productivity methods are switched around like fad diets because the underlying issue (anxiety and lack of satisfaction) is never corrected.

This becomes obvious once it comes to your attention that the people who are supposed to be experts in this, apply their productivity towards selling more productivity. Which is delightfully perverse.

In reality, if you are embarking in projects that are important to you, you rarely need that many reminders and get by with the simplest system (such as what you are describing) or none at all. All trivial other tasks can fit in a single list. And the things in your personal life that are actually meaningful, such as fulfilling relationships, are not something you can Zettlekasten your way into beyond setting a reminder for some birthdays.