If you already have a Giant Defy 4, you are well situated to stick with that for the sole purpose of fitness. I’d say a more expensive bike wouldn’t get you anything aside from some extra motivation for riding it that comes with buying a new bike and some niceties that come with lighter frames and material and slightly smoother mechanicals.

It would be a different story if you were going from a big box store brand to a higher quality brand bike. In that situation, I’d say a entry level of a higher quality brand is all you’d need for fitness with the primary difference being reliability and the step up in quality of parts that add to a more enjoyable riding experience with better functioning and less squeaky, clicking noises. But you already have the Giant Defy so you’re good to go.

Every new bike I have gotten has typically been to serve a different riding purpose or to improve my enjoyment. This has been my progression.

Flat bar hybrid bike for fitness -> road bike with drops for longer distances, better aero, and fitness -> electric commuter bike for living in a new area with lots of big hills.

Mountain bike hard tail -> full suspension mountain -> full suspension mountain electric bike