Never used the Nikon mirrorless system - used their latest DSLRs with the 500/5.6 PF and absolutely great AF. You are very fortunate, as I am, to have a wife who shares your interest in bird photography. I am paranoid about having back-ups for everything except wives. So, it's a good idea for your wife to have the same make gear as you to have a back up body and telephoto, share batteries, chargers etc. May I recommend that a Canon body with the RF 100-500mm? It's of similar weight and, although shorter, far more versatile than the 600/6.3.
Thanks Alan, for your nice reply. Yes, I am a lucky man, obviously like you (not only because my wife shares my love for photography). In fact, she "infected" me with photography many years ago (not birds back then, more people, street, landscape). She got her first camera as a young child, I always say, she was born with a camera in her hands. When we were a young couple, she was mostly the photographer and I was the draughtsman at her side, with a pencil and a sketchbook, who sometimes took out his already very vintage Kodak Retina IIIc to make a few images.
Regarding Nikon, she was several times close to changing to Canon, because she noticed that my gear often performed better and was much more rugged (I only once needed a service with my 7D, she several times, broken mirror boxes, dead AF drives, dead buttons), but she also has an extended film gear from Nikon and still loves these cameras, in particular her FM-2 bodies. That keeps her in the system (she made me a gift with an FM-2 many years ago that turned me into a Nikonian for a while).
The RF 100-500 would be too short for her, and I made her the 600mm prime as a gift because we nearly always are shooting at the longest focal distance, typical for birders. In fact, she can shoot her old Sigma 500mm f/4.5, a 3 kg lens, + TC hand-held with her D500, and she is petite with delicate arms and hands (half Chinese). I am always stunned but she uses a technique she learned from ballet dancing, when dancers have to hold their arms straight away from the body for extended moments. It is a special way to use the shoulder joint, I learned. But I thought, since we aren't that young anymore, that the Z 600 f/6.3 would be a good investment in the future when she doesn't want to carry up to 10 kg of glass, cameras, and mono/tripod in/on a backpack anymore. The alternative would be Nikons popular and much cheaper Z 180-600mm Zoom, but that lens isn't as sharp @ 600mm as the prime, and in particular adding a TC erodes the image quality visibly, as I've seen in reviews.
Generally, sharing Canon gear wouldn't help us much, since we both use comparable gear when we are shooting wildlife, macro or street etc. side by side. So we would need a double gear anyway, maybe except some special lenses like my EF 85mm f/1.2 (II). Sharing batteries and chargers wouldn't help us, too, because we need a lot of batteries anyway during a day trip (switching to ML cameras boosted that need, as you know), and in the night we need more than one charger per person anyway to get all of our batteries loaded for the next day, when we are on an extended photo trip.
