My wife and I are looking for a mirrorless full-frame birding/landscape combo. The desire to avoid the mirror rules out Nikon and Canon, which have a poor offering of native mirrorless long telephoto lenses. Trying to keep the costs down, we rejected Sony a9, and consider Sony a7iii + the 200-600 zoom lens. Is this a good combination? In particular I am interested in any feedback about the autofocus performance in low light (since low light shooting is the main reason for us to bother with a full frame camera).