Swings and roundabouts really. For me, the R5 does a great job and has very good ergonomics, but the A1 *possibly* has a slight edge when it comes to subject acquisition and tracking. The R5 is a bargain, the A1 is overpriced (but so IMO is the R3, especially since the launch of the Z9).
I've physically handled both, and they seem comparable in build quality. I don't know how they compare optically, as I've only seen the results from the 200-600mm, which are excellent. The price difference is huge, but I won't be getting the Sony simply because I can't justify the overall cost of switching systems, and as I said, I'm pretty happy with the R5. I guess what I'm really whining about is the choice of focal lengths. For bird photography the extra 100mm at the tele end is really valuable, and the short end far less so. The Sigma 150-600mm Sport in EF mount is probably the best match for me, but it does seem to have a few reliability issues.
Yes, a lighter lens would be nice (although perhaps not the Sigma 150-600mm Contemporary, which seems like the Sports version to get very mixed reviews), but I already have EF 100-400mm L and RF 800mm F11 for handheld work. What I want is to be able to encompass as closely as possible the focal length range of both of those lenses so that I'm not constantly swapping back and forth between lenses when shooting from a safari jeep in dusty Africa (although I swapped lenses maybe a dozen times each day in those conditions a couple of weeks ago, and didn't get a single dust speck on the sensor of my R5).