I used a Canon EOS R10 with a Tamron SP 150-600mm F/5-6.3 Di VC USD G2 at 600 mm and f/8.0. Larger apertures aren't that sharp with this lens, unfortunately.
Yeah, anything above f/8.0 tends to be a bit too soft for me at 600 mm. At the lens' lowest focal length of 150 mm, using f/6.3 tends to yield the sharpest results. Also remember that the larger the aperture, the smaller the number (f/6.3 is a larger aperture than f/8.0).
The effect is greatly exaggerated by the focal length, yes. The background is mostly trees that are like 20 metres away.
And yeah, photographing birds can be a bit tricky. So fast!