Step 1 - Get a remote contracting job where you are the sole engineer on a specific part of the product, like a mobile app or web service.

Step 2 - Work real hard on building a large undocumented code base. Get a reputation for speed and responsiveness.

Step 3 - Gradually do less and less while billing the same hours. Truthfully tell them that the code base has gotten so complex changes take a lot longer. Rely on your previous reputation to keep them comfortable with that.

Your client will likely accept all this at face value. Even if they get frustrated, no one will want to take on your large mass of undocumented code.

You can probably milk this as long as the product remains funded. The major risks are product cancellation or worse, high product success that leads them to want to invest more into development. Barring those two unlikely events you likely have years of coasting ahead of you.