b. Flexible builder

When you need arbitrary content to be smooth, here it goes:

SmoothBuilder(
  builder: (context, child) => ...,
  child: ...,
);

Indeed, it has almost identical signature as classical Flutter widgets such as AnimatedBuilder.

The whole app will run at normal FPS (may be janky), while the thing inside the builder callback will be 60FPS smooth. Therefore, for example, put animations inside the builder callback.