Advice needed on best use/settings - on full zoom using 200-600mm to capture a single, static bird at distance, the center focus area seems too large and will frequently focus on a point around or behind the bird such as branches or reeds etc. leaving the bird unclear. Is there a way to reduce the size of the center focus or should I be using a different setting? Advice appreciated - I'm still very much learning.

I'd use Focus Area: (Expand) Flexible Spot: S instead of Center. Smaller focus area and more control over where to focus.

https://helpguide.sony.net/ilc/1710/v1/en/contents/TP0001653124.html